i

PREVALENCE OF VISION CONDITIONS IN A

SOUTH AFRICAN POPULATION OF

AFRICAN DYSLEXIC CHILDREN

by

SAMUEL OTABOR WAJUIHIAN

Submitted in partial fulfillment of the requirement

for the degree of MASTER OF OPTOMETRY in the

Discipline of Optometry in the Faculty of Health Sciences

University of KwaZulu-Natal Durban

SUPERVISIOR Professor Kovin Naidoo

APRIL 2010

ii

DECLARATION

I hereby declare that this thesis is my own original work It is submitted to the University

of KwaZulu-Natal in partial fulfilment for the degree of Master of Optometry

SAMUEL OTABOR WAJUIHIAN

SIGNATURE helliphelliphelliphelliphelliphelliphelliphelliphelliphellip

DATE APRIL 2010

iii

ACKNOWLEDGEMENTS

There are at times that this project seemed to be a lsquonever ending journeyrsquo and many people have encouraged me greatly I wish to express my profound gratitude and acknowledgement of the contributions of the following people who have given me support in different ways

Professor Kovin Naidoo (my supervisor) for his guidance throughout the entire research process

The examiners for their critical evaluation of the study

My family for their patience throughout the time I was not there for them Special thanks to my wife Joyce for all her encouragements right from the beginning of the project To my parents for giving me the discipline never to give up no matter how rough things may be

Carrin Martin for her invaluable support in editing this thesis Members of staff of the International Center for Eye Care Education Sandro Thaver for the statistical analysis Mirashnee Rajah and Diane Wallace for their continued administrative and moral support

All members of staff of the Optometry Department University of Kwazulu Natal for their assistance in different ways

Celia G of ILAMO for assistance with referenced articles

Department of Education for granting permission to have access to the schools

The Principals Parents and Learners of schools (Khulangolwazi special school and Addington Primary schools) Durban for their cooperation

Authors of the numerous studies I consulted whose studies upon which the foundation of this study was based

The University of KwaZulu-Natal for initial financial support

iv

Abstract

Dyslexia is a neurological disorder with genetic origin that affects a personrsquos word

processing ability their spelling writing comprehension and reading and results in poor

academic performance As a result optometrists are consulted for assistance with the

diagnosis and treatment of a possible vision condition Optometrists are able to assist with

treatment as part of a multidisciplinary management approach where optometric support is

necessary International studies have indicated that up to 20 of Caucasian school children

are affected by dyslexia while there are no similar figures for African children Studies have

been done to assess the extent of visual defects among Caucasian dyslexics but not among

African dyslexic children The aim of the study is therefore to determine the prevalence of

vision conditions in an African South African population of dyslexic school children and to

investigate the relationship between dyslexia and vision

The possible relationship between dyslexia and vision conditions has been recognized as an

important area of study resulting in research being conducted in many countries Studies

have been undertaken by optometrists and ophthalmologists who differ in their approach

and attitude on how vision conditions affect dyslexia A review of the literature revealed

three broad areas of vision that may impact on reading ability these being acuity defects

binocular vision and ocular pathology Acuity defects consist of visual acuity and refractive

error Areas of binocular vision evaluated in the literature include near point convergence

heterophoria strabismus accommodative functions vergence facility and reserves

Hyperopia was the only vision variable that was found to be consistently associated with

difficulties with reading but not causally while findings on other variables were

inconclusive However all the studies acknowledged the complexity of the condition and

the need for a comprehensive multidisciplinary management approach for its diagnosis and

management

The study was undertaken in the city of Durban South Africa using a case-control study of

two groups of African school children between the ages of 10 and 15 Both study groups

consisted of 31 children of normal intelligence who were matched in gender race and

v

socio-economic status The case group attended a school for children with learning

disabilities while the control group attended a mainstream school At the time of the study

only one school catered for African children with learning disabilities and only 31 of its

pupils were diagnosed with dyslexia Ethical approval was obtained from the University of

KwaZulu-Natal permission to undertake the study in the identified schools was obtained

from the Department of Education and the school principals consented on behalf of the

learners as it was not always possible to reach the individual parent

The researcher (an optometrist) visited both schools by appointment where rooms were

made available to do the testing and the tests were explained to all participants The

LogMar Acuity Charts were used to assess visual acuity and static retinoscopy was used to

assess refractive error Binocular vision was tested using the cover test for ocular alignment

the Hirschberg test for strabismus RAF rule for near point of convergence 2 D flipper

lenses for accommodation facilities Donderrsquos push up methods using the RAF rule for

amplitude of accommodation plus and minus lenses for relative accommodation monocular

estimation technique for accommodation posture and prism bars for vergence reserves

Ocular pathology was assessed using a direct ophthalmoscope

The dyslexic group presented with the following Refractive errors hyperopia 65

myopia 65 astigmatism 10 anisometropia 65 remote near point of convergence

33 esophoria at near 3 exophoria at near 95 accommodative infacility 54 and lag

of accommodation 3928 The dyslexic group had relatively reduced fusional reserve

compared to the control group

The control group presented with the following Refractive errors hyperopia 3

astigmatism 13 anisometropia 65 remote near point of convergence 48 esophoria at

near 0 exophoria at near 0 accommodative infacility 33 and lag of accommodation

4193

The prevalence of a remote NPC was higher in the control group than in the dyslexic group

and there was a statistically significant difference between the two groups NPC break

vi

(p=0049) and recovery (p=0046) The prevalence of poor binocular accommodation

facility at near was higher in the dyslexic group than in the control group and there was a

statistically significant difference between the two groups (p = 0027)

Vision defects such as hyperopia astigmatism accommodation lag convergence

insufficiency poor near point of convergence and accommodative infacility were present in

the dyslexic pupils but they were no more at risk of any particular vision condition than the

control group This study provided the prevalence of vision conditions in a population of

African dyslexic children in South Africa the only vision variable that was significantly

more prevalent in the dyslexic population being the binocular accommodation facility at

near although the study was unable to find a relationship between dyslexia and vision The

statistically significant difference may not imply clinical significance due to the small

sample size However it is recommended that any vision defects detected should be

appropriately compensated for as defective vision can make reading more difficult for the

dyslexic child

The sample size may have been a limitation however this was comparable with studies

reviewed most of which had sample sizes of less than 41 Due to the range of possible

ocular conditions that could affect dyslexia it is recommended that a larger sample size be

used to ensure more conclusive results Testing for relative accommodation with a

phoropter would provide more accurate results and accommodation facility and fusional

reserves would be better assessed with suppression control The study provides information

and an indication of research needs regarding the prevalence of vision defects in an African

South African population of dyslexic children

vii

TABLE OF CONTENTS

CONTENTS PAGETITLE PAGE iDECLARATION iiACKNOWLEDGEMENTS iiiABSTRACT ivTABLE OF CONTENTS viiLIST OF APPENDICES xiLIST OF TABLES xiiLIST OF FIGURES xiiiLIST OF PUBLICATIONS FROM THIS THESIS xiv

CHAPTER ONE INTRODUCTION 1

11 Background and Rationale for the Study 112 Aim 213 Objective 214 Research Questions 215 Significance of study 316 Goals of Study 317 Null Hypothesis 318 Research (alternative) Hypothesis 419 Scope of Study 4110 Assumptions 5111 Study Outline 5112 An overview of Dyslexia 5113 The Reading Process 8114 Eye Movement in reading 9115 Characteristics Features of Children with Dyslexia 10116 Implications of Dyslexia for learning 11

1161 Reading and Comprehension 121162 Handwriting 121163 Proof-reading 12

117 Prevalence of Dyslexia 12118 Aetiology of Dyslexia 13

1181 Neurological Factors 141182 Genetic Factors 151183 Peri-natal Factors 151184 Substance Use during Pregnancy 151185 Environmental Factors 161186 Nutrition 16

119 Diagnosis of Dyslexia 171191 Exclusionary Method 17

viii

1192 Indirect Method 171193 Direct Method 18

120 Functional Imaging Techniques used in the Diagnosis and Study of Dyslexia 191201 Positron Emission Tomgraphy (PET) 191202 Functional Magnet Resonance Imaging (fMRI) 19

121 Classification of Dyslexia 201211 Primary Dyslexia 211212 Secondary Dyslexia 21

122 Sub-types of Dyslexia 211221 Dysphonetic (auditoryphonetic) 211222 Dyseidetic (visual-spatial) 211223 Dysnemkinesia (motor) 211224 Dysphonetic-Dyseidetic 23

123 Theories of Dyslexia 231231 Phonological-Deficit Theory 231232 Cerebellar Theory 251233 Visual Deficit Theory 261234 Auditory Processing Theory 27

124 Cultural Differences in Dyslexia 28125 Treatment Intervention 30126 Role of Optometry in the Multidisciplinary Management of Dyslexia 30

1261 Case History 311262 Refractive and Binocluar Functions 311263 Visual-Motor Skills 311264 Visual-Spatial Analysis 321265 Short-term Visual Memory (visual-motor recall) 321266 Auditory Analysis Skills 32

127 Optometric Treatment Options 331271 Correction of Vision Defects 331272 The use of Coloured Overlay 33

128 Potential Mechanism for Benefit from Tinted Lenses 35129 Summary of Chapter One 37

CHAPTER TWO LITERATURE REVIEW 39

21 Introduction 3922 Visual Acuity 4023 Refractive Errors 47

231 Myopia 47232 Hyperopia 49233 Astigmatism 49234 Amblyopia 50235 Anisometropia 50

24 Near point of Convergence 53

ix

25 Heterophoria 5526 Strabismus 5827 Accommodation Functions 59

271 Amplitude of Accommodation 59272 Accommodation Facility 61273 Relative Accommodation 64274 Accommodation Posture 64

28 Vergence Facility 6529 Fusional Reserves 66210 Ocular Pathology 68211 Summary of Chapter Two 73

CHAPTER THREE METHODOLOGY 75

31 Introduction 7532 Research Design 75

321 Sampling Design 75322 Study population and Participants 75323 Inclusion Criteria for Dyslexic Group 76324 Exclusion Criteria for Dyslexic Group 76

33 Ethical Clearance 7734 Testing Protocol 7735 Study Setting 7736 Data Collection Technique 78

361 Visual Acuity 79362 Refractive Error 79363 Near Point of Convergence 80364 Strabismus 80365 Heterophoria 81366 Evaluation of Accommodation Functions 83367 Fusional Reserves 85368 Ocular Health 87369 Vergence Facility 87

37 Statistical Analysis 8738 Summary of Chapter Three 89

CHAPTER FOUR RESULTS 91

41 Introduction 9142 Prevalence of Vision Defects 93

421 Visual Acuity 91422 Refractive Errors 92423 Near Point of Convergence 94424 Ocular Health 96

x

425 Heterophoria 96426 Accommodation Functions 97427 Fusional Reserves 101

43 Summary of Chapter Four 105

CHAPTER FIVE DISCUSSION 107

51 Introduction 10752 Prevalence of Vision Defects 107

521 Visual Acuity 109522 Refractive Errors 116523 Heterophoria 116524 Near point of Convergence 118525 Accommodation Functions 120526 Fusional Reserves 126527 Ocular Pathology 130

53 Summary of Chapter Five 130

CHAPTER SIX CONCLUSION 132

61 Introduction 13262 Summary of Findings 134

621 Visual Acuity 134622 Refractive Errors 134623 Near Point of Convergence 135624 Heterophoria 135 625 Accommodation Functions 136 626 Fusional Reserves 137627 Ocular Pathology 137

63 Implications of the Study Findings 138 64 The Study Strengths 138 65 Limitations of the Study 139 66 Recommendations 140

661 Future Research 140662 General Recommendations 140

67 Conclusion 141

REFERENCES 143

xi

LIST OF APPENDICES PAGE

APPENDIX A Clarification of Terms 156APPENDIX B Request for Consent from Department of EducationAPPENDIX C Letter of Consent from Department of EducationAPPENDIX D Letter of Consent from Addington Primary schoolAPPENDIX E Visual Acuity Conversion Table

xii

LIST OF TABLES

TABLE TITLE PAGE

11 Characteristic Features of Dyslexia 1012 Features of the Magnocelular and Parvocellular systems 2721 Conclusions from Major Studies on Dyslexia and Vision 7031 Data Collection Tools and Measurement Outcomes 7832 The Diagnostic Criteria for each Test Variable 8741 Descriptive Statistics for Visual Acuity 9242 Descriptive Statistics for Refractive Error 9443 Descriptive Statistics for Near Point of Convergence Break

and Recovery Points 9544 Descriptive Statistics for Heterophoria 9745 Descriptive Statistics for Accommodative Posture 9846 Descriptive Statistics for Binocular Accommodation Facility 9947 Descriptive Statistics for Amplitude of Accommodation 10048 Descriptive Statistics for Relative Accommodation 10149 Descriptive Statistics for Base-in Vergence Reserves at Distance 102410 Descriptive Statistics for Base-in Vergence Reserves for Near 102411 Descriptive Statistics for Base-out Vergence Reserves at Distance 103412 Descriptive Statistics for Base-out Vergences at Near 103413 Descriptive Statistics for Dyslexic and Control Groups

for all Variables 104

xiii

LIST OF FIGURES

FIGURE TITLE PAGE

41 Prevalence of Visual Acuity 9242 Prevalence of Refractive Errors (total) 9343 Prevalence of the Types of Refractive Errors 9444 Prevalence of NPC Break 9645 Prevalence of Phoria at Near 9646 Prevalence of Accommodation Lag 9847 Prevalence of Reduced Binocular Accommodation Facility at Near 9948 Prevalence of Relative Accommodation 101

1

CHAPTER ONE INTRODUCTION

11 Background and Rationale for the Study

Optometry has long been involved in the subject of vision and learning As primary eye care

practitioners optometrists receive referrals from parents teachers psychologists and other

professionals who usually seek the optometristrsquos advice about whether a childrsquos vision problem

may be contributing to or is responsible for poor academic performance

According to Scheiman and Rouse1 learning which is the acquisition of knowledge or skill can

be achieved through study experience or instruction and that between 75 and 90 of what a

child learns is mediated through the visual pathways As vision plays a major role in reading and

the learning process any defects in the visual pathway disrupt the childrsquos learning

ldquoThe three interrelated areas of visual functions are 1

1 Visual pathway integrity including eye health visual acuity and refractive status

2 Visual efficiency including accommodation binocular vision and eye movements

3 Visual information processing including identification and discrimination

spatial awareness and integration with other senses

Several studies2-6 have been conducted to investigate the relationship between vision and dyslexia

on Caucasian populations while very few have been done on African subjects Consequently

most inferences and conclusions on dyslexia are based on research conducted in Caucasian

populations Naidoo7 reported that inferences drawn from studies in Caucasian populations may

not be appropriate for the African population as the lsquoAfrican Eyersquo differs from the Caucasian eye

For example epidemiological studies have reported variations in refractive errors where the

prevalence of myopia is higher among Caucasian persons than in African persons8 9 while

African populations had higher open angle glaucoma prevalence than Caucasian persons10

This study attempts to contribute to the literature by examining the prevalence of a broader

spectrum of vision variables in dyslexic school children in order to investigate a possible

2

relationship between dyslexia and vision in African school children in South Africa A search of

the literature revealed that very little research has been conducted in Africa the only one being

conducted in 1997 by Raju11 Raju11 investigated oculomotor functions in dyslexic school

children in South Africa and reported that 65 of the normal readers exhibited normal

oculomotor control as compared to 11 of the dyslexics Furthermore 54 of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills while 12 of the

controls displayed the same

12 Aim

The aim of the study was to determine the prevalence of vision conditions in a South African

population of African dyslexic children and to study the relationship between vision and dyslexia

13 Objectives

1 To determine the distribution of visual acuity disorders among African dyslexic children

2 To determine the distribution of refractive errors among dyslexic children

3 To determine the distribution of phorias among dyslexic children

4 To determine the distribution of strabismus among dyslexic children

5 To determine the distribution of accommodation disorders among dyslexic children

6 To determine the distribution of vergence disorders among dyslexic children

7 To determine the distribution of ocular pathology among dyslexic population

8 To compare these findings to a similar group of non-dyslexic children

14 Research Questions

1 What is the prevalence of vision conditions among African dyslexic children of age range

10 and 15 years

3

2 What is the relationship between dyslexia and vision

15 Significance of the Study

The study will attempt to

1 Identify high-risk ocular conditions among dyslexic children which might influence the

need for certain examinations or that could affect the childs ability to learn to read

2 Relate findings of this research work to that of previous researchers

3 Add to the knowledge base in the area of vision and dyslexia

16 Goals of Study

1 To obtain quantitative information on the prevalence of visual conditions in dyslexic

children in the African population

2 To obtain information on the relationship between vision disorders and dyslexia

3 To enable improvement of eye care for children with dyslexia

17 Null Hypothesis

1 There will be no statistically significant difference between the mean visual acuity of the

dyslexic group and of the control group

2 There will be no statistically significant difference between the mean refractive errors of the

dyslexic group and of the control group

3 There will be no statistically significant difference between the mean phoria in the dyslexic

group and of the control group

4 There will be no statistically significant difference between the strabismic mean angle of

deviation of the dyslexic group and of the control group

5 There will be no statistically significant difference between the mean accommodation

functions (amplitude facility posture) in the dyslexic group and of the control group

4

6 There will be no statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

1 8 Research (alternative) Hypothesis

1 There will be a statistically significant difference between the mean refractive errors in the

dyslexic group and of the control group

2 There will be a statistically significant relationship between the mean visual acuity of the

dyslexic group and of the control group

3 There will be a statistically significant difference between the mean of phoria of the

dyslexic group and of the control group

4 There will be a statistically significant difference between the strabismus mean angle of

deviation of the dyslexic group and of the control group

5 There will be a statistically significant difference between the mean accommodation

functions of the dyslexic group and of the control group

6 There will be a statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

19 Scope of Study

1 This study is limited to the assessment of visual functions in dyslexic and normal readers

The visual functions investigated include visual acuity refractive error ocular alignment

near point of convergence accommodation functions and vergence reserves The

assessment of eye movement was not part of the study

2 Screening for and diagnosing dyslexia was not part of this study The school

psychologistsrsquo reports were relied on in selecting subjects who qualify to participate in this

study The information was obtained from the learnersrsquo school files 3 The scope of vision parameters investigated is limited only to the prevalence and

relationship between these variables in dyslexic and control groups as outlined in the

hypothesis Correlation analysis of different variables was not part of this study

5

110 Assumptions

In this report dyslexia is used synonymously with ldquodevelopmental dyslexiardquo ldquoreading

disabilityrdquo and reading disorderrdquo and will be used to describe children who find it extremely

difficult to learn to read despite having normal intelligence appropriate educational opportunities

and absence of emotional disorders Children so defined have reading age that is two or more

years behind their chronological age12

111 Study Outline

This thesis is divided into six chapters Chapter one indicates the rationale of the study provides

an overview of dyslexia and details its purpose objectives significance and goals Chapter Two

(the literature review) gives a detailed review of literature in the area of dyslexia learning

disabilities and vision with particular emphasis to vision variables as outlined in the hypothesis

to be tested This is followed by methodology in Chapter Three which details the research

methodology in which a case-control study was done in two schools in Durban The areas of

vision variables evaluated include visual acuity and refraction binocular vision and ocular health

evaluation Chapter Four (Results) presents an analysis of the results obtained from the study

Chapter Five (Discussion) discusses the study findings and their meaning Chapter Six presents

conclusions from the study based on the study results indicating the implications applications

and recommendations for future study and the study limitations

112 An overview of dyslexia

Some children find it difficult to learn to read despite having normal intelligence appropriate

educational opportunities and absence of emotional disorders Their reading age is two or more

years behind their chronological age Such children are referred to as being dyslexics Where

there is an associated deficit in intelligence the condition may be described as generalized

learning disability12

6

The term dyslexia is derived from the Greek words dys meaning hard or difficult and lexia

from the word lexicos which means pertaining to words so dyslexia means difficulty with

words-either seen heard spoken or felt as in writing13

The World Federation of Neurologists13 and the International Classification of Diseases-1014

defined dyslexia as ldquoa disorder manifested by difficulty in learning to read despite conventional

instruction adequate intelligence and socio-cultural opportunity It is dependent upon

fundamental cognitive disabilities which are frequently constitutional in originrdquo

Dyslexia is a complex disability which affects different aspects of reading The acquisition of

reading related skills are coordinated by visual motor cognitive and language areas of the brain

Consequently dyslexia can result from deviation of normal anatomy and function of cognitive

and language areas in the brain 15

Dyslexia refers to a cluster of symptoms resulting in people having difficulties in specific

language skills particularly reading15 The mystery in dyslexia as a syndrome lies in the fact that

dyslexics have ldquodifficulty to learn to read despite having average or above average intelligence

adequate auditory and visual acuity absence of frank brain damage no primary emotional

problems and have adequate educational instructionsrdquo 16

Dyslexia is a developmental disorder that shows in different ways at different developmental

stages As documented by Krupska and Klein 13 dyslexia is a learning disability characterised by

problems in expressive or receptive oral or written language that manifest initially as difficulty in

learning letters and letter sound association which is followed by difficulty in learning letters and

reading words accurately which consequently results in impaired reading rate and written

expression skills (handwriting spelling and compositional fluency) Some dyslexics find it

difficult to express themselves clearly or to fully understand what others mean when they speak

The effects of dyslexia reach well beyond the classroom as it can affect the persons self-image

It makes them feel dumb13 17 Dyslexia describes a different kind of mind often gifted and

productive that learns differently An unexpected gap exists between learning aptitude and

achievement in school 18

Many dyslexics are creative and have unusual talent in areas such as art athletics architecture

7

graphics electronics mechanics drama music or engineering 18 Some dyslexic learn best with

a multi-sensory delivery of language content which employ all pathways of learning at the same

time seeing hearing touching writing and speaking18 It is primarily not a problem of

defective vision although underlying vision deficit has been known to exaggerate the dyslexics

reading problems 18

According to Critchley19 ldquothe child otherwise normal as regards to intellectual status and

emotional stability finds an isolated difficulty in mastering the significance which lies behind

printed or written symbols and of necessity finds it extremely hard to align the verbal

components of speech (as emitted by the mouth and as received by the ear) with the empirical

characters which are set out on paper in one form or anotherrdquo Dyslexics are unable to acquire

written language skills through ordinary learning and teaching methods and often fail to progress

in education19

A dyslexic individual displays a characteristic pattern of decoding (recognising) and encoding

(spelling) difficulties with written words20 Interestingly dyslexics may not have overt

difficulties with spoken language yet do have marked difficulties with written languages21 The

difference between written and spoken language is that written language is seen while spoken

language is heard This fact may explain why vision has always been implicated on the topic of

dyslexia 21

Dyslexics experience difficulties with visual-verbal matching necessary to establish word

recognition ability Even when a clear single visual image is present the dyslexic may be unable

to decode the printed word in some cases This may be related to the fact that the difficulty that

dyslexics have is at a much more fundamental stage in the reading process so the role of vision

function is quite different in dyslexia than in other types of reading difficulty22 (which may be

peripheral such as hyperopia leading to difficulty in reading) The two main situations in which

the term dyslexia now commonly applies are when the reader has difficulty decoding words (that

is single word identification) and the second is encoding words (spelling) The second type is a

frequent presentation in optometric practice that is when the reader makes a significant number of

letter reversals errors (example b-d) letter transposition in word when reading or writing

(example sign for sing) or has left-right confusion 23

8

113 The Reading Process

Language development entails four fundamental and interactive abilities listening speaking

reading and writing while learning to read involves a complex system of skills relevant to visual

(the appearance of a word) orthographic (visual word form) phonological and semantic

(meaning of word and phrases) processing and a variety of behaviors such as letter naming letter

perception word recognition and comprehension each of which uses a different part of the brain

Reading is an interpretation of graphic symbols24-26

For some children the ability to break a word into its smaller parts a task crucial in reading is

extremely difficult Dyslexia is a localized problem one involving the sounds and not the

meaning of spoken language Speaking is natural while reading is different it must be learned

To be able to transform the printed letters on the page to words that have meaning the letters

must be connected to something that already has inherent meaning- the sounds of spoken

language In order to learn to read children must learn how to link the printed letters on the page

to the sounds of spoken language and understand that words are made up of sounds and that

letters represent these sounds or phonemes24- 26 In other words learning to read requires that the

child be able to break the word into the individual components of language represented by letters

and to be able to tell the difference between the individual phonemes that make up a word

Learning to read requires that the central principle behind the alphabet is understood that is

words are made of phonemes and in print phonemes are represented by letters Phonemes are

the shortest units of sound that can be uttered in a given language and that can be recognized as

being distinct from other sounds in the language For example the word cat has three phonemes

ca ah and ta Blended together the sounds form the word rsquocatrsquo24-26 When one speaks the

sounds are blended ttogether and are said one cat 24-26 To learn to read a child must learn that

there are three separate sounds This is difficult for children with dyslexia The inability to break

word into its parts is the main reason why children with dyslexia have trouble learning to

read24- 26

Furthermore dyslexic children find it difficult to bring the print to language (such as when asked

to read what they just wrote) For example a child can copy the letters w-a-srdquo correctly but

9

when asked what was written a child with dyslexia may reply saw The problem in this case

may not be related to vision but rather one of perceptual skills of what the child does with a word

on a page The brain mechanism of going from print to language is phonologically based 24-26

Reading acquisition builds on the childrsquos spoken language which is already well developed

before the start of formal schooling Once a written word is decoded phonologically its meaning

will become accessible via the existing phonology-to-semantics link in the oral language system

Thus the childrsquos awareness of the phonological structure of sound of speech plays a major role in

the development of reading ability 24-25

114 Eye Movement in Reading

For a person to be able to read a letter or word the eye has to first identify the written material

Normally the eyes scan across the line of words on the page stopping to fixate at various points

then making short jumps from one position to the next (some saccade eye movements are

involved) Typically there are around five fixation pauses in a line Sometimes the eye will

regress and go back to a previously passed over word element A reader views the text mainly

with the controlling or reference eye whilst the other eye diverges and converges within fine

elements27 A sophisticated mechanism27 (a connection of the visual receptors of the retina with

the visual nervous pathway to the brain) 28 ensures control of this drift and prevents diplopia from

being consciously observed These drifts and subsequent recovery can be clearly seen by a

number of techniques including infrared observation of the eyes27

The minimum time a person can spend on fixation and move to the next is around 250

milliseconds27 For skilled readers the average saccade length is 7-9 letter spaces and the

duration of saccades is of the order of 25-50 milliseconds29 while the average fixation pauses last

between 200-250 milliseconds27 29 Thus our eyes are still for some 90 of the time while

reading and are subject to large changes in acceleration and deceleration Even a good reader

will regress and go back 10 to 20 of the time27 29

The textual image seen by the eyes is imaged onto both maculae and transmitted to both cerebral

10

cortex where it is first received in the primary visual cortex (Brodmanrsquos area 17) with some

initial processing of colour and tone occur Progressively higher levels of processing occur in

adjacent areas 18 and 19 with the highest level of visual language processing taking place in the

angular gyrus (area 39) The angular gyrus is central to reading since it is here that hearing

speech and vision meets and the written word is perceived in its auditory form The auditory

form of the word is then processed for comprehension in Wernickersquos area as if the word had been

heard25 27

115 Characteristic SignsSymptoms of Children with Dyslexia

Dyslexia is heterogeneous in nature and there is no single pattern of difficulty that affects all

people Dyslexic children frequently show a combination of characteristics (Table 11)

Table 11 Characteristic SignsSymptoms of Dyslexia 16 30-31

Reading Visionminus Holds book too close

minus Word reversals

minus Skips complete words

minus Re-reads lines

minus Points to words

minus Word substitution

minus Sees double

minus Poor comprehension in oral reading

minus Might see text appearing to jump around on a page

minus Unable to tell difference between letters that look similar in shape such as o and e

minus Unable to tell difference between letters with similar shape but different

orientation such as b and p and q

minus Letters might look jumped up and out of order

minus Letters of some words might appear backwards eg bird looking like drib

11

Spelling

minus Omission of beginning or ending letters

minus Can spell better orally than written

minus Letter reversals

minus Wrong number of letters in words

Difficulty with Reading

minus Difficulty learning to read

minus Difficulty identifying or generating rhyming words or counting syllables in

words (Phonological awareness)

- Difficulty with manipulating sounds in words

minus Difficulty distinguishing different sounds in words (Auditory discrimination)

AuditoryVerbalminus Faulty pronunciation

minus Complains of ear problems

minus Unnatural pitch of voice

minus Difficulty acquiring vocabulary

minus Difficulty following directions

minus Confusion with beforeafter rightleft

minus Difficulty with word retrieval

minus Difficulty understanding concepts and relationships of words and sentences

116 Implications of Dyslexia for Learning

Dyslexia affects people in different ways and depends on the severity of the condition as well as

the effectiveness of instruction or remediation The core difficulty is with word recognition and

reading fluency spelling and writing Some dyslexics manage to learn early reading and spelling

tasks especially with excellent instruction but later experience their most debilitating problems

when more complex language skills are required such as grammar understanding textbook

material and writing essays The dyslexic syndrome therefore affects reading and learning in the

12

following ways263034

1161 Reading and Comprehension

One major difficulty dyslexics have is the time it takes to read and understand a text This makes

reading an extremely difficult task and they may report that they get tired and may get headaches

after reading for a short while30 There is also difficulty in assimilating what they have read even

when they know the words Multiple-choice questions are usually difficult for them as they find

it difficult to recognise words out of context263034

1162 Handwriting

Dyslexics have problems with handwriting since for some forming letters takes concentration

that their ideas get lost and their written expression suffers30

1163 Proof-reading

Dyslexics with visual processing difficulties usually have difficulty in identifying errors in

their own writings Reading difficulties make it uneasy for them to see their own errors in

expression and sentence structure grammar26 30

117 Prevalence of Dyslexia

Estimates of the prevalence of dyslexia in Caucasian populations vary from 5 to 20 in the

general population16 Pavlidis 31 reported that about 1 to 3 of the total population of children

suffers from dyslexia and that 20 to 30 of those cases classified as ldquogeneral reading failures

are probably dyslexics Evans32 reported a prevalence of 4 - 5 in children and stated probably

more boys than girls are dyslexic Park33 documented that up to 15 of the school-age

population has some degree of dyslexia with a ratio of four boys to one girl The large variability

in the prevalence reported by different authorities may be attributed to the differences in the

diagnostic criteria and the cut-off point applied to the psychometric tests14 Dyslexia is the most

common and the most researched type of learning disability34

13

A literature search revealed one study in Egypt that reported the prevalence of specific reading

difficulty in 2nd and 3rd grades in elementary school population to be 1 and the male to female

ratio to be 27 to 1 The authors concluded that the prevalence was low compared to that reported

in western countries and that the difference may be due to the way the Arab language is written

and read 35

Contrary to the above reports that males tend to be more dyslexic than females Guerin Griffin

Gottfried and Christenson 36 studied dyslexia subtypes and severity levels with respect to gender

differences The authors36 concluded that

when objective criteria such as the discrepancy between achievement (in

reading andor spelling) and intelligence or direct assessment of coding

skills are used as criterion and non-referred or random samples are

assessed the ratio of males to females identified as dyslexics approaches

11 When identification depends on referrals from parents and or

teachers more males than females may be detected because of the nature

and intensity of boysrsquo behavioural reactions to the disability

According to Shaywitz and Shaywitz34 good evidence based on sample surveys of randomly

selected population of children now indicates that dyslexia affects boys and girls comparably

118 Aetiology of dyslexia

The literature contains diverse theories (not less than four major theories) on the aetiology of

dyslexia probably due to the complex nature of the subject but several authors13-15 26 37-42 have

maintained that it is a neurological disorder with a genetic origin Despite decades of

multidisciplinary research neuropsychology brain anatomy and neuro-imaging 37-42 the specific

causes of dyslexia are still unknown The leading assumption however is that dyslexia

fundamentally stems from subtle disturbances in the brain and other possible explanations of the

cause of dyslexia such as language processing difficulties are based on the neurological etiology

14

The aetiological considerations in dyslexia are

1181 Neurological Factors

As documented by Habib 37 the concept of the neurological basis of dyslexia was first mentioned

independently by a Scottish ophthalmologist Hinshelwood in 1895 and a British physician

Morgan in 1896 Both found similarities of certain symptoms of dyslexia in some children

Dyslexia was then described as ldquovisual word blindnessrdquo The earlier understandings for the

neurological basis of dyslexia came from neuro-pathological studies of brains from dyslexic

persons According to Habib37 it was first reported by the French neurologist Dejerine in

1891 that damage to the left inferior parieto-occipital region (in adults) resulted in variable

degree of impairments in reading and writing suggesting that the left angular gyrus may play a

role in processing the ldquooptic images of letterrdquo It was subsequently thought that impaired reading

and writing in the young dyslexic patients could be due to abnormal development of the same

parietal region which was damaged in adult alexic patients 37

Another perspective 13 37 is that the dense distributions of ectopias (the presence of neural

elements in places where they are not supposed to be found) all over the cerebral cortex

(particularly in the perisylvian language areas) resulted in alteration of brain organisation One

such alteration in dyslexia is the lack of asymmetry in a language-related cortical region called

the planum temporale (an auditory area that lies on the superior surface of the temporal lobe) In

normal subjects the platinum temporale is usually larger in the left hemisphere However the

dyslexics showed symmetry of the planum temporale 13 37 Furthermore these ectopias may

reduce cortical connectivity as suggested by recent positron emission tomography and magnetic

resonance imaging studies 37

Another speculation was that the development of ectopias could be due to foetal hormonal

(possibly testosterone) imbalances during late pregnancy which could also account for the

possible male predominance in dyslexia 15 30 37 38

The maturation lag which is believed to be due to the slower development of the nerve fibres of

the corpus collosum (bundle of cells connecting the left and right hemisphere of the brain) has

also been theorized to be a cause of dyslexia A delay in the growth of the fibres linking the two

15

hemispheres would result in affected children being physiologically incapable of tasks which

require hemispheric organisation This leads to the inadequate development of the language

functions 12 37 However recent neuro-imaging studies 39-42 have shown that impaired reading of

alphabetic scripts (such as in the English language) is associated with dysfunction of the left

hemisphere posterior reading systems primarily in the left temporo-parietal brain regions

1182 Genetic Factors

Genetic considerations in dyslexia are derived from the fact that dyslexia runs most often in

families However it is speculated that different forms of dyslexia may occur within the same

family which also may means that it seems likely that the inheritance is indirect 15 31 However

genetic markers have been identified in chromosome 18 38 and chromosomes 6 and 15 15 In

fact chromosomes 2 (DYX3) 6 (DYX2) and 15 (DYX1) in dyslexics have been reported to be

inherited in an autosomal dominant mode of transmission 15 Furthermore Ramus 43 stated that

ldquogenetically driven focal cortical abnormalities such as ectopias and microgyri in specific areas

of left perisylvian cortex involved in phonological representations and processing are the

primary cause of dyslexia which is consistent with anatomical studies of dyslexic brains showing

loci of cortical abnormalities functional brain imaging studies showing that the very same areas

are involved in phonological processing and show abnormal activation in dyslexicshelliprdquo

1183 Peri-natal Factors

Peri-natal factors may manifest as problems during pregnancy or complications after delivery

More importantly it can also happen that the mothers immune system reacts to the foetus and

attacks it as if it was an infection It has also been theorized (based on partial evidence) that

dyslexia may occur more often in families who suffered from various autoimmune diseases (such

as rheumatoid arthritis Gravesrsquo diseases multiple sclerosis systemic lupus Sjogrenrsquos

syndrome)3137

1184 Substance Use during Pregnancy

16

Drugs such as cocaine (especially in its smokable form known as crack) seem to affect the

normal development of brain receptors of the foetus It has been documented that mothers who

abuse substance during pregnancy are prone to having smaller babies and such infants are

predisposed to a variety of problems including learning disorders Thirdly alcohol use during

pregnancy may influence the childrsquos development and lead to problems with learning attention

memory or problem-solving There is also a risk of foetal alcohol syndrome (characterised by low

birth weight intellectual impairment hyperactivity and certain physical defects) 30

1185 Environmental Factors

Environmental factors are risk factors for the development of reading disabilities 44 Reading

disabilities which are due to environmental influences have been referred to as non-specific

reading disabilities 20 or pseudo- dyslexia as the reading disorder is secondary to extraneous

factors and genetic factors play an important role 45 ldquoEnvironment factors play a significant role

in the level to which neurological impairments manifest themselves Encouraging nurturing and

stimulating surroundings can reduce the impact of risk factors on learning performance and life

skillsrdquo 44 Environmental circumstances which may profoundly influence the dyslexic child

include parental attitude the role of multilingualism in the home attention problems the

drawback of frequent changes of school the personality of the teacher and the childs emotional

reaction to his difficulties 152030

It seems that the influence of environmental factors may best be described as a potential risk

factor than a causative factor in dyslexia as there are diverse views on the relationship between

environmental influences and dyslexia According to Grigerenko46 socioeconomic level

educational opportunity and home literacy level are major environmental risks for the

manifestations of developmental dyslexia hellip the ldquomagnitude of the effects was not strong enough

to view them as powerful causal factorsrdquo

1186 Nutrition

Nutrition plays an important role in bridging the gap between genetic constitution and a childs

potential for optimal development Nutrition is a basic requirement for the maintenance of

17

growth and overall development while malnutrition and under-nutrition are associated with

disorders in the normal development of intelligence perceptual maturation and academic

achievements 47 Studies conducted by Grant Howard Davies and Chasty 48 showed that there

is an association between dyslexia and low concentration of zinc Zinc is necessary for normal

brain development and function and essential for neurotransmission44

119 Diagnosis of Dyslexia

The first stage in the diagnosis of dyslexia is with awareness by parents or educators that a

problem in reading exists The initial point of assessment starts with the medical practitioner

(possibly a paediatrician) who conducts a complete medical examination and obtains a

comprehensive health history A correct diagnosis of dyslexia starts with a very well taken case

history especially good awareness of the childrsquos developmental history This may give

indications of any medical condition that may be contributing to reading difficulties If indicated

the child may be referred for a neurological examination If dyslexia is suspected the physician

may then refer the child for further evaluation and treatment by a specialist in psycho-educational

assessments In most cases the diagnosis of dyslexia is usually performed by the psychologist

The Psychologists conduct psychometric assessments which include assessment of intelligence

quotient (IQ) various aspects of reading performance spelling mathematics sequential memory

and other aspects of cognitive and emotional development49 50 The major purpose of the

diagnostic process is to isolate the specific difficulties associated with dyslexia and to suggest

appropriate educational intervention 49 50

There are three methods for diagnosing dyslexia

1191 Exclusionary Method

In the exclusionary method the diagnosis of dyslexia is made when all factors involved in a

nonspecific reading disability (reading problems resulting from factors such as intellectual

disability dysfunction of hearing or vision inadequate learning experience socio-cultural

deprivation primary emotional problems poor motivation) are excluded 2051 The problem with

18

the exclusionary method of diagnosing dyslexia is that dyslexia cannot be diagnosed until after

the child has been failing in school for almost two years By this time constant failure may have

produced a negative attitude towards school and psychological problems 31

It provides a very

limited description of the features of the disorder Dyslexia is often defined in terms of the

discrepancy between reading achievement and chronological age or grade level failing to

consider the possible overlap between some of the intelligence quotient tests and reading tests 52

According to Snowling 53 IQ is not strongly related to reading and so the use of IQ in defining

dyslexia is inappropriate stating that many children with low IQ can read perfectly well and that

the discrepancy definition (based on the difference between reading achievement and

chronological age) of dyslexia cannot be used to identify younger children who are too young to

show a discrepancy

1192 Indirect Method

The second method of diagnosing dyslexia is the indirect approach A diagnosis is sought by

attempting to associate neurological soft signs such as finger ldquoagnosiardquo with reading failure The

drawback with this method is that many dyslexics do not manifest neurological soft signs 20

Another type of the indirect method involves the analysis of cognitive tests An example is the

Wechsler Intelligence Scale for Children (Revised) (WISC-R) WISC-R which compares ldquoverbalrdquo

versus ldquoperformancerdquo 20

1193 Direct Method

The third method which seems most reliable is the direct method which involves the use of

characteristic decoding and spelling patterns to determine the specific dyslexia It has the

additional advantage of addressing the fact that dyslexia is heterogeneous 20

19

120 Functional Imaging Techniques used in the Diagnosis and Study of

Dyslexia

There are functional imaging techniques for diagnosing and studying brain activity in dyslexia

These techniques include positron emission tomography (PET) Magnetic resonance imaging

(MRI) and computed tomography (CT) The two major techniques include

1201 Positron Emission Tomography (PET)

PET is a non-invasive imaging technique that uses radioactively labeled compounds to

quantitatively measure metabolic biochemical and functional activity in living tissue It

assesses changes in the function circulation and metabolism of body organs 54 55 The PET is

a sophisticated nuclear medicine technique that looks directly at cerebral (brain) blood flow

and indirectly at brain activity (or metabolism) The most commonly used labels are

flourophores which emit light when stimulated with light of the appropriate wavelength and

radio nuclides which emit gamma rays or beta particles when they decay56 The dose of

radionuclide injected is minute and does not pose any significant hazard Various isotopes are

used depending upon the brain region and function studied54

Unlike MRI or CT imaging which primarily provide images of organ anatomy PET measures

chemical changes that occur before visible signs of disease are present on CT and MRI images 54 While PET is most commonly used in the fields of neurology oncology and cardiology

applications in other fields are currently being studied 57

1202 Functional Magnetic Resonance Imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a noninvasive usually painless procedure that

uses a powerful magnetic field radio waves and a computer to produce detailed pictures of

organs soft tissues bone and virtually all other internal body structures The images can then be

examined on a computer monitor or printed The fMRI does not use ionizing radiation57 58

Functional magnetic resonance imaging (fMRI) is a relatively new procedure that uses MR

imaging to provide a picture of the brains active rather than its static structure The fMRI

20

machines measure the level of oxygen in the blood through a technique called blood level-

dependent (BOLD) contrast The two different states of hemoglobin-oxygen-rich oxyhemoglobin

and oxygen-poor deoxyhemoglobin differ in their magnetic properties The huge fMRI magnets

are sensitive to changes in the concentration of deoxyhemoglobin As neural activity increases

blood flow to the vasculature of the brain presumably increases altering this concentration The

more active a brain area is the more blood flows to it Thus fMRI can provide a moment-by-

moment movie of brain activity

Studies using fMRI are beginning to shed light on dyslexia Functional magnetic resonance

imaging (fMRI) is similar to MRI as both procedures use a similar scanner The difference is that

the fMRI uses somewhat more sophisticated hardware and software that allow it to capture brain

changes (mainly blood flow) as a person performs a specific cognitive task The functional

magnetic resonance imaging (fMRI) scanner measures where blood flows through the brain

during certain tasks26

Earlier studies of dyslexia were based on histo-pathological studies According to Habib37 the

fMRI has an added advantage that cortical anatomy of the dyslexic brain can be demonstrated

thus answering the criticism addressed to results from pathological findings

120 Classifications of Dyslexia

Dyslexia can be classified into two broad categories according to its presumed aetiology The

following methods of classification have been documented by Helveston59

1 Primary dyslexia (specific developmental dyslexia)

2 Secondary dyslexia

1211 Primary Dyslexia

This is considered to be caused by a specific central nervous system defect This neural defect is

thought to be located in the angular gyrus of the dominant hemisphere It is often hereditary and

21

may affect more boys than girls In primary dyslexia the decoding process involved in language

system is affected and reading becomes extremely difficult for the child However intellectual

tasks that do not involve reading text are unaffected 59

1212 Secondary Dyslexia

There are two types of secondary dyslexia59 Endogenous and Exogenous

a Endogenous Dyslexia this results from pathological changes in the central nervous

system secondary to trauma or disease such as childhood meningitis hydrocephalus

with brain damage and porencephalus59 Other medical causes include prematurity

congenital hydrocephalus encephalitis traumatic brain injury and lead or

methylmercury poisoning50

b Exogenous Dyslexia this results from intellectual deprivation usually limited to lack of

adequate educational opportunity or motivation59 or due to low intelligence socio-

cultural deprivation primary emotional problems sensory impairment (visual or

auditory) poor motivation or attention problems20 Reading disability due to these

mentioned factors have been referred to as non-specific reading disability20

1 22 Sub-types of Dyslexia

When children first start to read they learn to recognise simple words by their shapes (sight

analysis) building their sight vocabulary This is followed by learning how to analyse

complicated words by breaking them down into their sound components (phonetic analysis)

The dynamics of the two processes (Sight-analysis and Phonetic analysis) are important in

explaining how words are decoded based on the neurologic-behavioural model documented by

Christenson et al 20 and Evans 21 Dyslexia can be divided into the following subtypes 20 21

22

1221 Dysphonetic (auditoryphonetic)

The individual has a minimal dysfunction involving the Wernickes area Readers have an

extremely difficult time developing phonic word analysis or decoding skills Their typical

reading errors are wild guesses (lsquolikersquo for lsquolittlersquo) errors picking words with similar shapes

(horse for house ) and semantic substitutions (home for house) Their typical spelling errors

are phonetically inaccurate (reffor rough) This type of reader persists in using whole-word

approach and will guess at unfamiliar words rather than using their word analysis skills While

they do not often continue to reverse letters and words after the age of eight they often make

spelling reversals (For example interver for inventor and wirters for writers) Typically

they often look at the first and last letter and the length of the word and then guess (monkey for

money or stop for step)20 21

1222 Dyseidetic (visual-spatial)

In dyseidetic type of dyslexia dysfunction occurs in the angular gyrus The individual have high

ability in phonic word analysis They often have a difficult time learning the letters of the

alphabet They usually have a very low sight vocabulary poor visual memory poor perceptual

skills and letter and word reversals (dig for big or saw for was) Their reversals often last

past the age of eight and include both reading and writing reversals This type of child is often a

very slow reader who often will try to sound out familiar as well as unfamiliar words They spell

the word as it sounds By first identifying the learning style of these children probably during the

case history the optometrist may know the childs strengths and weaknesses Vision therapy can

then be designed to fit each childs needs Enhancing the childrsquos visual perceptual and eye

tracking skills plus having the schools teach these children according to their strengths enable

them to achieve their full potential in school 20 21

122 3 Dysnemkinesia (motor)

Dysnemkinesia is a less serious form of dyslexia and it involves minimal dysfunction of the area

of the motor cortex involved in letter formation Dysnemkinesics can be distinguished by their

23

frequent letter reversals such as d for b as in doy for boy Additional dyslexic types occur

when combinations of the three basic types arise such as dysphoneidesia dysnemkineidesia

dysnemkinphonsia and dysnemkinphoneidesia20 21

1224 Dysphonetic-Dyseidetic

Dysphonetic- dyseidetic reader has a weakness in both whole-word and phonic analysis Their

reading errors include wild guesses (fish for father) and word reversals (no for on) Their

spelling is usually phonetically inaccurate and often bizarre (rit for faster) 2021

123 Theories of Dyslexia

Assisted by the increasing body of knowledge obtained by neurophysiological and imaging

studies14 several theories have been proposed with the aim of characterizing the fundamental

processes underlying dyslexia14 These theories can be classified under four major frameworks

phonological visual cerebellar and auditory

1231 PhonologicalndashDeficit Theory

According to the phonological theory affected individuals have difficulties in perceiving and

segmenting phenomes leading to difficulties in establishing a connection between phonemes and

graphemes 14 The brain recognizes language in a hierarchical order The upper levels deal with

semantics (the meaning of words) syntax (grammatical structure) and discourse (connected

sentences) The lower levels of hierarchy deal with breaking sounds into separate small units

called phonemes (sound units) Thus before words can be comprehended at higher levels in the

hierarchy it has to be broken down into phonologic constituents that the alphabetic characters

represents15 26 An adequate reading development stems from some considerable level of spoken

language already acquired by a child in the early years of life Consequently failure to develop

the association between letters (grapheme) and sound (phoneme) has been considered to be a

major cause of reading and spelling impairment in most cases of developmental dyslexia60-62

24

Reading requires some skills of phonological processing to convert a written image into the

sounds of spoken language1 According to the phonologic-deficit hypothesis people with

dyslexia have difficulty developing an awareness that words both written and spoken can be

broken down into smaller units of sound and that in fact the letters constituting the printed word

represent the sounds heard in the spoken word2

According to Habib37 neuropsychological studies have provided considerable evidence

indicating that the main mechanism leading to reading difficulties is phonological in nature and

that it has been repeatedly demonstrated that the core deficit responsible for impaired learning to

read is phonological in nature and has to do with oral language rather than visual perception At

the brain level this cognitive deficit would arise from a congenital dysfunction of certain cortical

areas involved in phonology and reading60 At the neurological level it is usually assumed that

the origin of the disorder is a congenital dysfunction of the left-hemisphere perisylvian brain

areas underlying phonological representations or connecting between phonological and

orthographic representations60

Brain imaging studies in dyslexics in response to a phonological task indicate under-activation of

posterior brain regions (Wernickesrsquos area angular gyrus and striate cortex) and relative over-

activation in anterior regions (inferior frontal gyrus) These brain activation patterns provide

evidence of an imperfectly functioning brain system for segmenting words into their phonologic

constituents39 42 Since most dyslexics show deficits in phoneme processing the phonological

deficit is the most significant consistent marker of dyslexia and remains the predominant theory15343739 hence the focus on phonology in the majority of remedial programmes 63

Based on the phonological deficit theory the other symptoms of dyslexia are considered as

simple co-morbid markers that do not have a causal relationship with reading disability62

However proponents of the phonological theory typically contend that these disorders are not

part of the core features of dyslexia62 A major setback with the phonological theory is that it

does not account for symptoms such as impairment in visual perception and problems with motor

coordination that are found in dyslexics 15 and does not account for symptoms of dyslexia that are

unrelated to phonetic decoding difficulties such as problems with short term memory and visual

25

processing issues64

The two basic processes involved in reading are decoding and comprehension In dyslexia a

deficit at the level of the phonologic units impairs the ability to segment the written word into its

underlying phonologic elements Consequently the reader experiences difficulty initially in

decoding the word and then in identifying it The phonologic deficit is independent of other non-

phonologic abilities especially the higher-order cognitive and linguistic functions involved in

comprehension such as general intelligence and reasoning vocabulary and syntax are generally

intact A deficit in phonologic analysis and an ironical intact higher-order cognitive ability offers

an explanation for the paradox of otherwise intelligent people who experience great difficulty in

reading 26

According to the phonological model a deficit in a lower-order linguistic (phonologic) function

prevents access to higher-order processes and to the ability to draw meaning from text The

problem is that the person cannot use his or her higher- order linguistic skills to access the

meaning until the printed word has first been decoded and identified Suppose for example that

a man knows the precise meaning of the spoken word apparition however until he can decode

and identify the printed word on the page he will not be able to use his knowledge of the

meaningof the word and it will appear that he does not know the words meaning 26

1232 Cerebellar Theory

The normal pattern of cerebellar asymmetry is anomalous in dyslexia 13 The cerebellar deficit

theory suggests that the automatisation of cognitive processes and motor control in the

cerebellum are disturbed in individuals with dyslexia14 15 62 Evidence for this theory comes

from poor dyslexic performance in coordination balance and time estimation The cerebellar

theory is related to cerebral dominance (greater proficiency of each cerebral hemisphere in

acquisition and performance of certain specific functions) and lateralization (one-sided location

of brain function)13

Physiologically the brain operates cross-laterally thus the left part of the brain controls the right

side of the body and vice versa The human brain is normally asymmetrical due to the

26

specialized functions in the left and right brain However the dyslexic brain appears to be more

symmetrical than the normal brain The consequence of this is that both sides of the brain may

be competing to handle language and therefore less efficient in handling processing language

The left hemisphere is especially important for the production of language The right hemisphere

also has some language capabilities but has no ability to analyse speech sound Secondly the

cerebellum plays a role in motor control and therefore in speech articulation It is postulated that

retarded or dysfunctional articulation would lead to deficient phonological representations

Furthermore the cerebellum plays a role in the automatization of tasks such as driving typing

and reading A weak capacity to automatize would affect among other things the learning of

grapheme-phoneme correspondences 13-15 62

1233 Visual Deficit Theory

For many years a major view was that reading difficulties are caused by dysfunction in the

magnocellular subsystem 65-67 The visual theory does not exclude the phonological deficit but

emphasizes a visual contribution to reading problems in some dyslexics The biological basis of

the proposed visual theory is derived from the division of the visual system into two sub-systems

which carry information to the visual cortex the transient and the sustained systems62

Anatomically the magnocellular pathway receives input from both rods and cones across the

retina and extends from ganglion cells in the retina to the two ventral layers of the lateral

geniculate nucleus (LGN) The large cell bodies in these two layers give the pathway its name It

projects via the visual cortex and the dorsal or ldquowhererdquo stream to the parietal cortex The

parvocellular (or sustained) system on the other receives input from cones and therefore mainly

the central retina and runs from retinal ganglion cells to the smaller cell bodies of the four dorsal

layers of the LGN It projects via the visual cortex and the ventral or ldquowhatrdquo stream to the

infero-temporal cortex and mediates colour vision and detection of fine spatial details61 The

magnocellular system is the first to respond to a stimulus and relies on further detail from the

parvocellular system (Table 12)

27

Table 12 Features of the Magnocellular and Pavocellular Systems2168

Magnocellular(transient) System Pavocellular (sustained) SystemLarge cells Small cells

Dim light Bright light

High contrast sensitivity Low contrast sensitivity

Low spatial frequency High spatial frequency

Peripheral Central

Right brain Left brain

Midbrain Cortical

Non-cognitive Cognitive

Emotional Logical

Hyperopia Myopia

Gross depth perception Stereopsis

Achromotopsia Colour vision

Fast transmission Low transmission

Sensitive to large stimuli Sensitive to small stimuli

The transient system is vital to the timing of visual stimuli and plays an important part in

detecting the movement of objects (motion perception) and ultimately their signals are used to

control eye and limb movements made in relation to visual targets1521 Impaired function of

magnocellular pathway will lead to destabilization of binocular fixation which results in visual

confusion and letters then appear to move around

Dyslexicsrsquo eyes are unsteady when they are attempting to view small letters hence their vision is

unstable they tend to make visual reading errors15 There is a tendency to superimpose letters on

top of each other and mis-sequence letters in a word1621 Evidence for magnocellular

dysfunction comes from anatomical studies of dyslexic individuals showing abnormalities of the

lateral geniculate nucleus1562 psychophysical and neuroimaging studies 62 64 showing decreased

sensitivities in the magnocellular range (low spatial frequencies and high temporal frequencies)

in dyslexics

28

A deficit in the magnocellular system slows down the visual processing system resulting in an

unstable binocular control inaccurate judgements of visual direction a tendency to superimpose

letters and mis-sequence of letters in a word16 21 Some studies65-67 have shown evidence in

support of the magnocellular deficit theory of dyslexia

1234 Auditory Processing Theory

The auditory processing theory specifies that the deficit lies in the perception of short sounds and

fast transitions (called the ldquorapidrdquo or ldquotemporalrdquo auditory processing deficit) Such a

characterization of the auditory dysfunction is consistent with the magnocellular theory since

magnocellular cells are particularly sensitive to high temporal frequencies60 62

Proponents of the auditory processing theory hypothesized that impaired perception of brief

sounds and transitions would be particularly detrimental to speech perception hence would

undermine the development of the childrsquos phonological representations Contrary to this

hypothesis Rasmus60 reported that recent studies have now adequately confirmed that there is

no reliable relationship between performance on rapid auditory processing tasks and speech

categorization and discrimination neither is there a reliable relationship between any auditory

measure (speech or non-speech) and more general measures of phonological skill or reading

ability even when assessed longitudinally A possibility is that the most auditorily impaired

dyslexics also have severely impaired phonology and reading although the reverse is not

necessarily true 60

Despite the fact that evidence exist in support of the theories and that some of the deficits found

in affected individuals are correlated with reading and spelling they may not be causally

associated with dyslexia However results from genetic research may have the potential to help

delineate which cognitive and neurophysiological processes are causally related14

124 Cultural Differences in Dyslexia

The recognition of dyslexia as a neuro-developmental abnormality has been influenced by the

belief that it is not a specific diagnostic entity because it has variable and culture-specific

29

manifestations 69-71 Recent neuro-imaging studies39-42 have demonstrated that impaired reading

of alphabetic scripts is associated with dysfunction of left temporo-parietal brain regions

However it has been reported by Siok et al in two separate studies69 70 that in contrast to the

assumption that dyslexia in different languages has a universal biological origin their study using

functional magnetic resonance imaging with reading-impaired Chinese children and associated

controls showed that functional disruption of the left middle frontal gyrus is associated with

impaired reading of the Chinese language (a logographic rather than alphabetic writing system)

The neural connections involved in reading and reading disorders may vary in different languages

because of differences in how a writing system links print to spoken language69 70 In Chinese

for example graphic forms (characters) are mapped to syllables which differ markedly from an

alphabetic system (such as English) in which graphic units (letters) are mapped to phonemes

These differences can lead to differences in how reading is supported in the brain69 70 Readers

of Chinese show relatively more engagement of visuo-spatial areas and left middle frontal

regions for verbal working memory presumably for recognizing complex square-shaped

characters whose pronunciations must be memorized instead of being learned by using letter-to-

sound conversion rules69 70

In two separate studies Siok Perfetti and Tan69 and Siok Zhen Charles Perfetti and Tan 70

studied language-related activation of cortical region of Chinese dyslexic school children using

the functional magnetic resonance imaging and found a reduced activation in the same left

middle frontal gyrus region in Chinese dyslexics Chinese dyslexics by contrast did not show

functional or structural differences from normal subjects in the more posterior brain systems that

have been shown to be abnormal in alphabetic-language dyslexics69-70 The authors concluded

that the results of the studies suggested that the structural and functional basis for dyslexia varies

between alphabetic and non-alphabetic languages and that the findings provided an insight into

the fundamental patho-physiology of dyslexia by suggesting that rather than having a universal

origin the biological abnormality of impaired reading is dependent on culture

The study by Paulesu Demonet Fazio and McCrory71 gave another perspective Using the

positron emission tomography to study brain activity of dyslexics from three countries (Italian

French and English) the authors found the same reduced activity in a region of the left

30

hemisphere in dyslexics from all three countries with the maximum peak in the middle temporal

gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus

Their study showed that there was a universal neuro-cognitive basis for dyslexia and that

differences in reading performance among dyslexics of different countries are due to different

orthographies Languages with transparent or shallow orthography (such as Italian) the letters

of the alphabet alone or in combination are in most instances uniquely mapped to each of the

speech sounds occurring in the language Learning to read in such languages is easier than in

languages with deep orthography (such as English and French) where the mapping between

letters speech sounds and whole-word sounds is often highly ambiguous 71

125 TreatmentIntervention

Individualized instruction of dyslexics emphasizing increased phonologic awareness can have a

favorable long-term effect on academic achievement Individualized instruction aimed at

increasing phonologic awareness decoding skills sight word vocabulary and reading

comprehension are particularly helpful to school aged children experiencing difficulties with

reading Most importantly intervention (provided by expert teachers) should be initiated early50

Reading disabilities require lifelong assistance and optimal management strategies differ

depending on the patientrsquos age and circumstances In early childhood the focus is on

remediation of reading often with an emphasis on increasing phonologic awareness Other

strategies include using audio books and modified homework assignments For secondary and

college students intervention focuses on accommodations which include extra time for reading

tape recorders in the classroom audio books instruction in word processing and the use of a

spell-checker (poor phonemic association also causes problems in spelling)3450

126 Role of Optometry in the Multidisciplinary Management of

Dyslexia

The role of the optometrist in the multidisciplinary management of dyslexia starts with an attempt

31

to identify characteristics which may indicate that the child may be dyslexic A complete visual

examination should be conducted Any vision anomalies detected should be corrected and a

prompt referral should be made to the appropriate specialist in the diagnosis of dyslexia and a

follow up should be made by the optometrist

A comprehensive visual evaluation should include1 72

1 Evaluation for a sensory problem or refractive anomaly

2 Evaluation for binocular vision problems

3 Evaluation for ocular pathology

4 An evaluation of visual motor skills (required for drawing and handwriting) eye movement

control skills and visual imagery skills

5 Evaluation of visual information processing including visual spatial skills

(rightleft discrimination) visual analysis skills (matching and discrimination skills)

The role of the optometrist in the management of reading dysfunctions is to identify existing

vision disorders which can be achieved by evaluating the following aspect of vision functions

1261 Case History

A detailed case history is paramount for children who present with reading dysfunction A

thorough investigation of the pre-natal peri-natal and post-natal risk factors is important to

establish the possibility of major developmental delay For example maternal history of

alcoholism may indicate foetal alcohol syndrome Prematurity (early birth date of less than 37

weeks or low birth weight of less than 2500g) is also a factor in later learning A working

knowledge that may not necessarily mean expertise in all related areas may be adequate for the

optometrist72

1262 Refractive and Binocular Functions

A thorough investigation and compensation for refractive and binocular vision functions should

be undertaken4472

1263 Visual-Motor Skills

32

VisualndashMotor integration is the ability to coordinate visual information-processing skills with

motor skills44 It is a very vital skill in academic achievement4472 Children suffering from visual-

motor dysfunction have difficulty copying written work accurately and efficiently Difficulty

with copying can reduce the speed and accuracy of writing which subsequently affects

learning4472 The Beery Test for Visual-motor Integration is particularly useful for the primary

care optometrist It requires only a few minutes to administer with an added advantage that the

scoring guide provides many examples of the pass-fail criterion for each item tested72

1264 Visual-Spatial Analysis

ldquoThe visual-spatial skills allow the child to make judgments about location of objects in visual

space in reference to other objects and to the individuals own body It develops from awareness

within the individualrsquos body of concepts such as left and right up and down and front and back It is important for good motor coordination balance and directional sensesrdquo 72 The Rosnerrsquos

Visual Analysis Skills programme is useful for the assessment of both visual-motor skill and

spatial analysis72

1265 Short- Term Visual Memory (Visual-Motor recall)

The visual memory test assesses the ability to recognize and recall visually presented material 4473 The ability to copy written material accurately is more or less a matching process when the

written materials are close (such as copying from workbook to paper) When the reading

material is separated from the actual visual-motor task such as copying from chalkboard to

paper the process becomes more complex An example of short-term visual memory skill

assessment test is the Getman-Henderson-Marcus Test of Visual recall72

1266 Auditory Analysis Skills

Screening for auditory-perceptual skill at the primary care level is essential as the ability to

analyze spoken language into separate sounds and sound sequence is directly related to reading

and spelling success Examples of tests to measure auditory perceptual skills are the Test of

33

Auditory Analysis Skills (TAAS) (which is easy to administer and score) and a more complex

one Test for Auditory-Perceptual Skills72

127 Optometric Treatment Options

The optometric treatment options for the correction of vision defects include the following

options

1271 Correction of Vision Defects

1 Compensation for refractive error

2 Treatment of visual efficiency deficits with lenses prisms and vision therapy

3 Treament of vision information processing deficits with vision therapy commencing with

visual spatial orientation then visual analysis and concluding with visual-motor

integration

4 Referral to another health care professional educational system and or psychologist

should be considered at any time if underlying physical or neurological problems cognitive

deficits or emotional disorders are suspected1 50 72

The expected outcome of an optometric intervention is an improvement in visual function with

the reduction of vision anomalies associated with reading which makes educational

intervention easy1 Solan47 proposed that the role of optometrists should not end with

correcting vision anomalies in dyslexic patients but that optometrists should participate in

community services to assist with the developmental environment of those whose genetic

make-up may predispose them to having reading difficulties

1272 The Use of Tinted Lenses and Coloured Overlays

The patients rely on professionals to attend to their vision care needs Consequently it is

imperative that we practice at the highest level of professionalism and ensure that anything we

prescribe for our patients are supported with credible research

As reported by Evans74 75 Olive Meares in 1980 suggested that some childrenrsquos perception of

text and certain cases of reading difficulties were influenced by print characteristics which

34

subsequently resulted in visual perceptual anomalies such as words blurring doubling and

jumping Subsequently Meares and Irlen proposed that ldquoScotopic sensitivity syndrome or

(Meares -Irlen syndrome) ndash a syndrome of visual symptoms and distortion can be alleviated

with coloured filters Meares and Irlen claimed that the coloured overlays improve reading

ability and visual perception increase sustained reading time and eliminate symptoms

associated with reading such as light sensitivity eyestrain headaches blurring of print loss of

place and watery eyes75 This type of symptoms is reported both by people who experience

frequent severe headaches of the migraine type as well as some people with dyslexia76

Following this initiative Irlen developed a proprietary system in countries like the United States

of America United Kingdom and Australia to detect and treat Meares-Irlen syndrome Meares-

Irlen syndrome is reported to be treated with coloured filters either coloured sheets (overlays)

placed on the page or colored lenses Computer users may attain a similar effect by changing the

colour of the screen background and or text while people that consistently work under the same

lighting conditions can benefit from the colour therapy by simply varying the colour of the

illuminating light75

The earlier studies of Irlen was criticized for its lack of published double masked studies with a

placebo control to support her claims that the colour that helped had to be specific for each

person76

According to Lightstone76 the Intuitive Colorimeter (developed by Professor Wilkins) of the

Medical Research Council in the United Kingdom was the first to be used to conduct a masked

trial With this instrument it was possible to determine the optimum colour for the spectacle lens

without placing lenses in front of the eyes and could be done in a way that the subject is unaware

of the exact colour they had selected At the end of the study it was felt that the use of colour

was credible and was then included as part of a routine assessment of people with specific

learning difficulties at the Institute of Optometry London76

As described by Lightstone76 the Intuitive colorimeter is basically a large box which the patient

or subject looks into The space that the subject looks at is entirely diffused by a coloured light

and so creates the effect of wearing coloured lenses There are no other colours in the field of

35

view and so no reference colours are available to help the subject decide the exact hue (colour) of

the light that they are viewing This encourages rapid colour adaptation The person operating

the colorimeter changes all aspects of the colour that the subject is viewing and the hue

saturation (depth of colour) and the brightness of the colour can be varied independently of each

other Although the procedure is subjective the technique shows whether there is a specific

colour that gives relief from the symptoms If the effect is placebo the results tend to be variable

and no specific colour can be selected as the optimum colour75 76

An advantage of the Intuitive colorimeter is that because of colour adaptation the person is

unaware of the exact color of the light shining on the text74 However a major demerit with

double-masked trial was that the results were purely subjective and there were no objective

measurement of any improvement with the colour 76

128 Potential Mechanism for Benefit from Tinted Lenses

Some theories have been proposed to be the mechanism of benefit from tinted lenses However

Evans 49 suggested that these hypotheses have not been able to account for the high degree of

specificity of the required colour which has been emphasized by Irlen and substantiated by

double-masked randomized placebo-controlled trials

The theories are

a Pattern Glare The basis of the theory on how pattern glare explains the benefits of tinted

lenses is that certain cells within the visual cortex are hypersensitive and so causes

discomfort while viewing patterns of repeating black and white lines on a page 75 Since

some of the neurons in the visual cortex are sensitive to specific colours varying the colour

of the illuminating light may change the pattern of excitation within the cortex This could

account for the benefit from specific coloured filters From this it has been hypothesized

that by changing the colour of the input the stimulus is transmitted to other cells that are

not hypersensitive and so prevent the distortions and discomfort occurring 7576

b Syntonics Syntonics is a colour vision therapy system used by some optometrist for the

treatment of several conditions including strabismus amblyopia and reading disorders

36

The therapy involves subjects viewing a light source covered by a colored filter The theory

is that the colour influences the endocrine system through the inferior accessory optic tract

The colour seems to be chosen on the basis of a hypothetical connection between an alleged

autonomic nervous system imbalance and the type of heterophoria Treatment is said to

result in improved visual fields7475

c Placebo Effect The placebo effect is one of the possible mechanisms for the benefit from

tinted lenses People may believe that a treatment works simply because it is claimed to be

effective74

Williams and Kitchner77 conducted a literature review on the use of tinted lenses and colored

overlays for the treatment of dyslexia and other related reading and learning disorders and

concluded that

1 there was evidence that the symptoms associated with Irlen syndrome are related to

identifiable vision problems such as binocular and oculomotor problems and that condition

returned to normal when treated with lenses prisms or vision therapy When patients

exhibiting the Irlen syndrome were treated with vision therapy the symptoms were relieved

and therefore did not demonstrate the need for colored lenses

2 most investigators did not control for the presence of vision anomalies

3 the results of prospective controlled research on the effectiveness of tinted lenses or colored

overlays varied

4 the results of testing used to determine the most appropriate color were not repeatable

5 the effects of spectral filters and colored overlays were not solely a placebo Colored

overlays and tinted lenses are not cures for dyslexia but may be useful reading aids for

some individuals with reading difficulty

6 there was a lack of agreement about the best ways to evaluate patients for the presence of

Irlen syndrome

The American Optometric Association78 issued a policy statement on the use of tinted lenses and

colored overlays which states as follows

37

1 Undetected vision problems may be a factor in individuals who exhibit the symptoms of

the Irlen Syndrome A comprehensive eyevision examination with particular emphasis

on accommodation binocular vision and ocular motor function is recommended for all

individuals experiencing reading or learning difficulties as well as those showing signs

and symptoms of visual efficiency problems

2 The American Optometric Association encourages further research to investigate the

effect that specifically tinted lenses and colored overlays have on visual function related

to reading performance

3 Vision problems are a frequent factor in reading difficulties Ignoring the role of vision

or inadequately evaluating the vision of individuals with reading problems is a disservice

which may prevent the person from receiving appropriate care

In concluding Lighthouse76 advices that until more is known about prescribing specific colored

lenses optometrists should be certain to offer the patient the best advice According to Wilkins 79

some 20 of unselected children in mainstream education (not simply those with reading

difficulties) find colored overlays of benefit and the benefit is pronounced in 5 Wilkins79

emphasized that ldquocolored filters do not provide a treatment for dyslexia ndash they provide a means of

avoiding the visual stress with which reading is sometimes associatedrdquo

129 SUMMARY

Dyslexia is a neuro-developomental condition that impacts on a persons spelling

comprehension writing and reading and not only limits their academic career but also impacts

on all areas of life that require these skills Its diagnosis management and treatment require a

multidisciplinary team as it can present with many characteristics which requires careful

classification While it is not always possible to identify the cause it is generally considered to

be a neurological condition with a genetic origin It affects up to one fifth of Caucasians with

limited studies being done on other ethnic groups including Africans

The aim of the study was therefore to investigate the prevalence of vision defects in an African

38

South African population of dyslexic schoolchildren with an attempt to identify high risk

visual conditions that may be prevalent It was hypothesized that there was no relationship

between vision defects and dyslexia

The reading process requires learning to transform printed letters on a page to words that have

meaning Words are made up of phenomes which are represented by letters and it is the

inability to break words into parts that result in the manifestation of dyslexia Its characteristics

include letter reversal faulty pronunciation difficulty with word retrieval and learning to talk

As all visual stimuli is transmitted through the eye there is an association between ocular

conditions and problems associated with letter words and reading hence the involvement of

optometrists The primary role of optometry is to identify and compensate for visual defects

that may constitute an impediment to reading Evaluation by an optometrist is therefore

important to assess and provide advice or devices where appropriate

Initial identification usually takes place by teachers or parents and may result in the child

being sent to a school equipped to deal with learning disabilities Psychologists diagnose

dyslexia using some range of psychometric tests The advent of imaging techniques such as

positron emission tomography and functional magnetic resonance has aided research in

dyslexia as these techniques enable the study of dyslexic brain activity as a reading task is

performed The two broad categories of dyslexia one being due to a genetic neurological

defect and the other being due to a secondary incident that impacts on the brain There are a

number of theories regarding the underlying causes the only theory related (still controversial)

to visual system being the visual processing theory

Chapter One provided the study aims objectives hypotheses to be tested background

information on dyslexia and rationale for the study while Chapter two will review the related

literature on dyslexia and vision

39

CHAPTER TWO LITERATURE REVIEW

21 Introduction

The relationship between vision and dyslexia has been a subject of controversy in optometric

and ophthalmologic literature A review of existing literature 1-32 on the area of vision and

dyslexia reveals that the studies conducted to investigate visual functions in the dyslexic

subjects do not establish a causal relationship between vision and dyslexia but tend to

establish associations between dyslexia and certain vision variables The literature review also

shows that there exists a dichotomy between the ophthalmologists and optometrists on the

subject of dyslexia and vision The difference in perspective can be seen from the style of

report in studies conducted by optometrists 5683-849198100-101 and by ophthalmologists 2-4 85 95

99102 Optometrists maintain that although vision may not be considered as a cause of dyslexia

compensating for visual defects make reading easier for the dyslexic child Ophthalmologists

hold the contrary view that vision does not in any way contribute to dyslexia Dyslexia has

also been thought to be a language-based learning disability80 Consequently from the speech-

language pathologistsrsquo perspective ldquoa child with a speech deficit may also have an emotionally

distorted perspective and a child whose classroom behaviour is hard to control may also be

unable to readrdquo The role of the psychologists is to perform psychometric assessments of the

dyslexic child32 The educator is primarily involved with special educational intervention for

the dyslexic child

Flax 22 proposed that the role of visual factors in dyslexia ldquodepends on the definition of vision

that is consideredPrime meaning that if vision is narrowly defined as visual acuity or clarity of

sight it is unlikely that there would be any relationship between vision and dyslexia If other

peripheral visual functions such as fusion convergence refractive errors and accommodation

are considered there is likely to be a relationship but then questioned the possibility of vision

being a causative factor Flax22 reiterated that the most important aspect of vision that is

closely linked to dyslexia is when vision is defined to include perceptual and integrative

aspects of vision It was concluded that ldquovision difficulties are part of the dyslexic

40

syndrome22

From a similar perspective Solan81 suggested that vision anomalies are contributory and

frequently hinder responses to educational intervention He emphasized that in cases where

vision anomalies are factors involved in the reading disability the effort required to maintain

clear single binocular vision decreases the efficient organization of the visual cognitive

response

This review has been structured in a way that an overview of a study will be made when it is

first cited Subsequently such studies will be cited only with references to the vision variable

being reviewed Only the studies that relate directly to the topic of the present research such as

visual acuity refractive errors near point of convergence ocular alignments accommodation

functions and vergence reserves are reviewed

It is also important to note that there are many variations in the results reported by different

authors In view of this the findings are thus presented on an individual basis Various studies

have attempted to study the relationship between dyslexia and vision and have examined the

different aspects of vision suspected of affecting reading ability However the major

drawbacks that plague the researches in the subject are Firstly intelligence quotient is a factor

to consider when defining dyslexia However only a few studies587-89 consulted have

considered intelligent quotient in their subjectsrsquo selection

Another area worth mentioning is the fact that some of the studies lacked control group which

made the reports difficult to assess and utilize In addition some researchers failed to classify

some visual functions such as binocular coordination and refractive errors thereby making it

difficult to evaluate the impact of a particular vision variable on reading ability Furthermore

some studies failed to indicate the eye examination techniques used in their protocol which

makes the replication of such studies difficultGiven the inherent limitations with studies

conducted to investigate vision in dyslexic children these studies are important in guiding the

teacher and clinicians on the appropriate intervention for the child

22 Visual Acuity

41

Vision plays an important role in the reading process as the ability to see printed material is

crucial in reading Reading efficiency and speed starts to decrease when the print size is less

than three times larger than an individualrsquos visual acuity threshold A marked bilateral

reduction in visual acuity can definitely limit reading performance but it is unknown if

deficient visual acuity characterizes students who have difficulty to read82

Available studies3-58384 that have attempted to see how visual acuity affects reading

performance are presented starting with the first three studies in this review that found a higher

prevalence of reduced visual acuity among dyslexics compared to normal readers

In 1994 Latvala Korhoenen and Penttinen 3 assessed the ophthalmic status of fifty five (55)

dyslexic and fifty (50) control subjects in an age gender and social class matched study in

Finland The subjectsrsquo school grade and intelligence quotient were not indicated Since

intelligence quotient is a major factor in the definition of dyslexia the lack of information on

intelligence quotient may lead to the questioning as to if the population studied were truly

dyslexics The authors assessed visual acuity using the Snellen chart and found that for the

control group all subjects had visual acuity of 07 (69) while in the dyslexic group two

children (36) had bilateral visual acuity of less than or equal to 07

A study to examine the lsquoophthalmologicalrsquo status of school children with dyslexia in Norway

was conducted by Aasved 4 in 1987 Visual examinations were performed on two hundred and

fifty nine dyslexic children The number of the participants from the control group was not

clearly stated The investigation techniques used was also not detailed As a result it may be

impossible for other researchers to replicate the study However Aasved 4 reported that the

dyslexic population had a higher prevalence of reduced visual acuity than the normal readers

did It was concluded that although there was no causal relationship between eye

characteristics and reading difficulties eye abnormalities should be corrected when detected in

dyslexic children

42

Evans et al 5 in 1993 studied some sensory and refractive visual factors in 39 dyslexic and 43

control subjects aged between seven years six months and twelve years three months in the

United Kingdom Participants from both groups had intelligence quotient of above 85 The

authors found that the dyslexic group had a significantly worse near and distance binocular

visual acuity the inter-ocular difference was not statistically different in the two groups and

the difference between the best monocular and the binocular visual acuity was not statistically

different between the two groups at distance or near They claimed that the ldquoreduced visual

acuity may not have been due to refractive defectsPrime Apart from refractive errors a reduced

visual acuity may arise from pathological conditions such as cataract and glaucoma

The following two studies reported similar prevalence of visual acuity worse than 69 In 1980

Hoffman83 investigated the incidence of vision difficulties in learning disabled children in the

United States of America Vision evaluations were performed on one hundred and seven

children with learning problems who were referred to the clinic for vision care from educators

and psychologists The subjectsrsquo mean age was eight years eight months As a comparison

(control) group twenty five patients between the ages of five and fourteen were selected from

the general optometry clinic The prevalence of visual acuity poorer than 69 was reported to

be 1028 among the learning disabled subjects The subjects from the control group were not

appropriately selected not representative enough and were not age-matched although the issue

of the comparison group being inappropriately selected was acknowledged by the author

Another limitation with the study was that no information was given on the eye examination

procedures followed This may be problematic as it makes comparison with other studies and

replication of the study difficult

Sherman84 conducted a study to ldquorelate vision disorders to learning disabilitiesrdquo in the United

States of America in 1973 Thirty-nine boys and eleven girls participated in the study The

children were diagnosed as having learning difficulties by the referring psychologists Their

43

ages ranged from six to thirteen years The technique used to measure visual acuity was not

indicated but it was reported that five children (10) had visual acuity poorer than 69

Other studies reported the prevalence of visual acuity to be similar between the control and

reading disabled group and include the following studies In 1985 Helveston Weber and

Miller85 (from ophthalmology) studied visual functions and academic performance in first

second and third grade children in the United States of America One thousand nine hundred

and ten children participated in this study The Snellen chart and the Jaegar print were used to

assess distance and near visual acuity respectively The authors reported that 94 of the

children tested had normal visual acuity and that visual acuity was not related to academic

performance in a positive way Although the study may be credited with a large sample size a

possible limitation with the application of the findings of this study was that the passfail

criteria were arbitrarily chosen by the examiners ldquoon the basis of previous clinical experiencerdquo

without references to other research studies Secondly the authors failed to present the results

of the statistical analyses of the relationships between vision functions and reading

Grisham and Simon82 conducted a review of literature on refractive error and reading process

and reported that there are abundant and reliable scientific evidence that shows a higher

prevalence of vision defects among reading disabled children than among normal readers It

was emphasized that there are cases where prescription of glasses has improved academic

performance Their analysis revealed no statistically significant difference in distance visual

acuity among normal and disabled readers but that near visual acuity may be an index which

distinguishes some children who are having difficulty to read and those who do not

It is important to note that since reading is a near point task a deficient near visual acuity may

be more contributory to poor reading than reduced distance acuity Consequently Grisham

and Simons82 stated that ldquodeficient distance visual acuity would not be expected to affect the

44

reading skills of children unless substantial reading instructions take place on the chalk board

It is possible that distance acuity is a factor in the early grades when reading is first

introducedrdquo

In 2007 Grisham Powers and Riles86 studied visual skills of 461 poor readers in California

high schools The participants mean age was 154 years and there was no control group in the

study Participants for the study had been identified by the schools as poor readers The vision

functions measured were visual acuity near point of convergence ldquoconvergence and

divergence and fusional rangesrdquo accommodative amplitude and accommodative facility No

information was given on IQ but it was mentioned that the basis for referral of students for

assessment was poor reading performance which was determined by the school and defined as

reading two grade levels or more below grade level Visual acuity was assessed using the

Snellenrsquos chart at six meters The authors reported that 568 of the participants had visual

acuity of 66 in both eyes 261 had visual acuity of 69 in each eye while 172 had visual

acuity of worse than 69 in either eye

Another comprehensive study conducted in 2007 was reported by Kapoula Bucci and

Jurion87 Convergence and divergence functions in 57 dyslexic and 46 non-dyslexic age-

matched children were studied in French dyslexic children The mean age of the dyslexic

subjects was 113 years the mean IQ was 105 and the mean reading age was 891 The mean

age for the control group was 107 years The IQ score of the control subjects was not

indicated The vision functions performed included visual acuity stereo acuity and cover test

near point of convergence and ldquomeasurement of convergence fusional amplitude (divergent or

convergent) Unfortunately the authors did not detail the testing procedures for the vision

functions investigated but reported the results of the study It was reported that all children had

normal visual acuity

In 2008 Alvarez and Puell 88 studied accommodation functions in school children with reading

difficulties in Madrid Spain Eighty seven poor readers (mean age 92 years) and 32 control

(mean age not indicated) participated in the study all in grades three through grade six

Relative accommodation amplitude of accommodation and accommodation facility was

assessed

45

The most current literature reviewed was reported by Bucci Gignac and Kapoula 89 in 2008

The authors studied poor binocular coordination of saccades in dyslexic children in France Of

particular interest to the current review is the assessment of visual acuity and refraction

(though was assessed under cycloplegia) heterophoria near point of convergence and fusional

vergence reserves The study participants comprised of 18 dyslexic children (mean age 114

years and mean IQ was 105) The control group comprised aged-matched non-dyslexic

children mean age 11 2 years There were no details on the method of assessment of visual

acuity and refraction but it was indicated that a cycloplegic refraction was conducted

However the authors reported that all children had normal visual acuity but the results for

refractive errors were not reported

In 1991 a study on stereopsis accommodative and vergence facility was conducted by

Buzzelli 6 in the United States of America Thirteen normal readers of average age thirteen

years three months and thirteen dyslexics average age of thirteen years four months

participated in the study Both groups were matched for gender age and intelligence quotient

The study design was comprehensive The Snellen chart was used to assess the visual acuity

Buzzelli6 found that the dyslexics did not perform worse than the control subjects in visual

acuity

Ygge Lennerstarnd Axelsson and Rydberg2 studied visual functions in a Swedish population

of dyslexic and normally reading children in 1993 Eighty six 9-year- old children matched to

controls with regard to age gender class in school and intelligence participated in the study

VA was assessed with ldquoordinary optotypes chartsrdquo They reported that the subjects from the

control group had a better visual acuity than the dyslexic group at both distance and near and

that the results showed statistically significant differences (distance at p =003) and (near at

p=0005)

A comparative study of dyslexic and normal readers using orthoptic assessment procedures

was conducted by Goulandris McIntyre Snowling Bethel and Lee90 The study was

conducted in 1998 in the United Kingdom and comprised 20 dyslexic and 20 chronological and

46

reading age-matched control participants A unique aspect of the study was that the distance

VA was assessed using the Cambridge Crowding Cards This test according to the authors

was devised to evaluate letter recognition acuity in conditions of lsquocrowdingrsquo when other letters

surround the target letter and that the test is designed to ensure that abnormalities of vision

which would not be revealed reliably using single targets were detected The near acuity was

assessed using the Snellenrsquos chart The use of different VA chart at distance and near may

raise a concern of standardization They reported that the prevalence of distance and near VA

(using a 69 criteria) was similar between the groups

In 1992 Evans Drasdo and Richards91 investigated optometric correlates of reading disability

in 10 children aged between eight and fifteen years who were referred to the optometry clinic

by the child psychologist The authors reported only the statistical values mean binocular

distance visual acuity of + 091 and mean binocular near visual acuity of + 089

47

23 Refractive Errors

In the study by Alvarez and Puell 88 no information was given on the protocol used to assess

refractive error but the refractive error results were reported For the poor readers mean

spherical equivalent refractive errors for the right and left eye were 020 06 and 020 06

respectively Fifty-seven (655) children were emmetropic In the control group mean

spherical equivalent refractive errors for the right and left eye were - 020 08 and - 014

080 respectively Twenty (645) children were emmetropic In contrast Evans et al 91

reported the mean spherical equivalent refractive error (by retinoscopy) of + 077DS for the

(right and left eye was similar)

231 Myopia

Myopia is characterized by a decrease in distance visual acuity Myopia would usually not be

expected to affect reading skills since students can make the necessary adjustments to their

reading positions However from a clinical standpoint unusually high myopia myopic

anisometropia and astigmatism could influence patients near performance and cause some

discomfort to the child There is no evidence in the literature that myopia is associated with

poor reading In the study by Alvarez and Puell88 57 of the children from the dyslexic

group were myopic compared to 194 from the control group

In 1987 Rosner and Rosner 93 conducted a retrospective comparison of visual characteristics of

data for two hundred and sixty one children with learning difficulties in the United States of

Ameria who had been identified by the schools as manifesting learning difficulties (LD) The

control group consisted of four hundred and ninety six children without learning difficulties

The subjects from both groups were of age range between six and twelve years The children

had been examined at the University of Houston Optometry Clinic The data collection

techniques used was not given but it may be assumed to be routine optometric eye examination

48

protocols because the study was conducted at the university optometry clinic The prevalence

of myopia (minus 050 dioptres) was reported to be 54 in the non-learning disabled patients

while 19 of the learning disabled patients were myopic

One limitation with this study however was that all subjects were patients of an eye clinic

where the two main reasons for referral are reduced visual acuity and school learning

problems Consequently the prevalence of visual defects in subjects selected from such a

clinical setting may have been higher than those from a non-clinical setting

In a review study conducted by Grisham and Simon82 the prevalence of myopia was lower in

the reading disabled group than among the normal readers Similarly Grosvenor 94 analysed

data from several studies and concluded that myopia is consistently associated with good

reading performance One limitation with this analysis however that is the conclusion drawn

from the review was based only on studies that found positive relationships between vision

anomalies and reading ability Grosvenor 94 however remarked that ldquomany authors failed to

find the relationship because they used ineffective methods

One of the earliest studies on the subject of vision and reading difficulty was conducted by

Eames 95 in 1948 in the United States of America He studied refractive and binocular vision

anomalies in one thousand (1000)rdquoreading failures one hundred and fifty (150) unselected

children and five hundred ophthalmic patientsrdquo Comparing the poor readers and the

unselected groups (control) he found the incidence of myopia to be same in both groups (4)

Myopia was defined as -100 diopter sphere There was no statistical analysis in that study and

the data collection techniques were vague although it was indicated that no cycloplegia was

used While the study may be criticized as not being current it is the major and early study to

give an insight into the subject of vision and reading disability

Eames95 and Shearer96 reported a similar prevalence of myopia (4) Shearer96 investigated

ldquoeye findings in children with reading difficultiesrdquo Two hundred and twenty two children

participated in the study The subjects were screened using the Keystone telebinoculars The

prevalence of myopia defined as - 050 dioptres sphere was 4 He concluded that vision

49

functions are not related to reading ability

232 Hyperopia

Hyperopia is often associated with poor reading performance probably through the mechanism

of deficient accommodation and poor motor fusion skills 82 A child with a relatively high

degree of farsightedness may not report blur vision at distance due to compensation for

hyperopia through the accommodation mechanism which also is used to create clear vision at

near This results in overuse of the accommodation system both optically and physiologically

and a subsequent fatigue or spasm of the accommodation system 97

One of the studies that classified refractive error in the literature was conducted by Rosner and

Rosner93 They reported that hyperopia appeared to be more prevalent (54) in learning

disabled than in the non-learning disabled children with a prevalence of 16

In the study conducted by Alvarez and Puell88 287 of the dyslexic group was hyperopic

compared to 161 from the control group Grisham and Simon82 in a literature analysis of

refractive errors and reading process reported that the prevalence of hyperopia among poor

readers is higher than among good readers and that the correction of hyperopia resulted in

improved performance

Grosvenor 94 reported that hyperopia is usually associated with poor reading Similarly in

Eamesrsquo study95 43 of poor readers had hyperopia while for the control group only 12 were

hyperopic The author gave no information on statistical analysis Shearer96 reported the

prevalence of hyperopia to be 16

233 Astigmatism

Clinically uncorrected moderate to high astigmatism can cause discomfort and lead to poor

reading performance at near distance Relatively few studies have attempted to study the

relationship between astigmatism and reading disability The prevalence of astigmatism

reported was higher in the dyslexic group compared to the control group in all the studies

reviewed 39395 These studies include the study by Latvala et al 3 who reported that 36 of

50

the dyslexic subjects were astigmatic (of greater than -100 diopter cylinder) and none from the

control group had astigmatism of that magnitude Eames95 reported the prevalence of

hyperopic astigmatism of more than -100 diopter cylinder among the poor readers to be 6

and 4 in the unselected group while the prevalence of myopic astigmatism was the same

(1) in both groups Rosner and Rosner 93 found a higher prevalence (30) of astigmatism

amongst the non-learning disabled subjects compared to the learning disabled group (27) In

Shearerrsquos 96 study of eye findings in children with reading difficulty the total prevalence of

astigmatism classified as myopic astigmatism (3) and hyperopic astigmatism (3) reported

was 6 There was no control group in this study In the study by Alvarez and Puell 88

astigmatism was detected in seven (8) and eight (92) right and left eyes respectively while

astigmatism was detected in five children (161) from the control group Ygge et al 2

reported that the prevalence of astigmatism was higher in the dyslexic group (28 left eye 25

) compared to the control groups (right eye 183 left eye 243) although there was no

statistically significant difference between the groups (p=025)

234 Amblyopia

Only two studies on the available literature consulted reported on the prevalence of amblyopia

both indicating a higher prevalence amongst the dyslexic group compared to the control group

In the study by Latvala et al 3 the prevalence of amblyopia in the dyslexic group was 36 and

none in the control group had amblyopia Although a detailed prevalence of refractive error

was not presented in that study it was reported that nearly all subjects had visual acuity of 07

In relating refractive error to amblyopia the prevalence of astigmatism in the control group

was 36 and none from the dyslexic group had astigmatism Consequently a low prevalence

of amblyopia may be related to fairly good visual acuities in both groups On the contrary

Rosner and Rosner 93 reported that 4 of the non-learning disabled patients had amblyopia

compared to the 3 found in the learning disabled patients Hyperopia is the refractive error

that is more often associated with amblyopia However this study presented a higher

prevalence of amblyopia (4) but a lower prevalence of hyperopia (16) in the non-learning

disabled patients However a difference of only 1 is not high enough to account for the

difference

51

235 Anisometropia

Anisometropia has been thought to be associated with poor reading skills probably through the

mechanism of poor sensory and motor fusion rather than reduced visual acuity It degrades

binocular coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 82

Three studies2395 reported findings on anisometropia Latvala et al 3 reported a higher

prevalence of anisometropia (of greater than or equal to one diopter) of 6 for the control

group than in the dyslexic group with 36 Similarly Ygge et al 2 found a higher prevalence

of anisometropia in the control group (158) than in the dyslexic group (94) Eames 95

reported that 13 of poor readers were anisometropic while 6 from the control group had

anisometropia According to Grisham and Simons 82 only few studies have considered

anisometropia to affect reading performance The authors concluded that there was insufficient

number of studies to draw firm conclusions from their review

Other studies2598 failed to classify refractive error according to the type These include

studies by Evans et al 5 who investigated some sensory and refractive factors in dyslexia The

distribution of refractive errors was found to be similar in both the dyslexic and control groups

The details of the participants profile was given earlier (section 21) The refractive errors

were assessed by distance retinoscopy and subjective refraction A cross cylinder was used to

measure astigmatism The spherical equivalent refraction was calculated by adding one-half

of the cylindrical components to the spherical component It was not indicated if refraction

was performed under cycloplegia Ygge et al 2 assessed refractive errors objectively using

streak retinoscopy under cycloplegia They reported that the distribution of refractive errors

were similar between the dyslexic and control groups

Sucher and Stewart 98 examined 136 fifth and sixth grade children comprising seventy two

with learning disability and 64 from the mainstream school as the control group The learning

52

disabled children were selected from a classroom of children attending special supervision and

instruction Children from both groups were matched by grade level No information on

intelligence quotient was given The testing protocol used to assess refractive error was not

indicated The authors reported a higher prevalence of refractive error in the dyslexic (208)

than in the control (94) group As indicated earlier refractive errors were not classified

according to the types in the three preceding studies 2598

Hoffman 83 reported the prevalence of refractive error to be 215 There was no report on the

control group and the techniques used to assess refractive errors were not indicated In the

study by Helveston et al 85 only 1 of the participants had refractive error when assessed

objectively with the retinoscope without the use of cycloplegia Refractive errors were not

classified according to the types (myopia hyperopia or astigmatism)

In 1991 Hall and Wick 99 conducted a study to investigate the relationship between ocular

functions and reading achievement in the United States of America One hundred and eleven

children (in grades one through grade six) participated in the study Eleven ocular functions

were investigated including visual acuity saccade eye movements accommodation facility

amplitude of accommodation accommodation posture heterophoria horizontal fixation

disparity vertical fixation disparity near point of convergence titmus and Randot stereopsis

Accommodation facility was measured using the flipper technique The amplitude of

accommodation was determined using the push up to blur technique Heterophoria fixation

disparity and near point of convergence were also measured but the techniques used were not

indicated The age range of the subjects studied was not indicated Using multivariate

correlation they found no statistically significant relationship between the ocular functions and

reading abilities as they noted that the acquisition of reading skills is given by many forces

which include the use of remedial instructions and language skills the role of peer pressure and

innate intelligence

53

24 Near Point of Convergence

Anomalies of convergence is one of the common problems encountered in the optometric

practice especially in the pediatric population and the reports on near point of convergence has

been presented comprehensively in the literatures reviewed

Grisham et al 86 measured the near point of convergence by asking the student if the clown face

target was seen as single when the examiner held it approximately 20 cm in front of the

studentrsquos eyes If so then the student was requested to say when the object appeared to double

as it moved closer to the face The examiner placed one end of an accommodative rule on the

studentrsquos forehead and judged where the two eyes diverged as the target moved toward the

studentrsquos face The measurement was repeated three times The authors found that 846 of

the students had near points at 8 cm or closer which they considered ldquonormalrdquo while 154

had near point of convergence of 9 cm or farther

In the study by Kapoula et al87 the NPC was determined by placing a small pen-light at

3040cm in the mid plane in front of the subject and moving it slowly towards the eyes until

one eye lost fixation The authors reported that the NPC was significantly more remote

(gt10cm) in dyslexics than in non-dyslexics (36 versus 15)

Bucci Gignac and Kapoula 89 followed similar protocol as in the study by conducted Kapoula

et al 87 The near point of convergence was determined by placing a small pen-light at 3040

cm in the mid plane in front of the subject and moving it slowly towards the eyes until one eye

lost fixation It was reported that the NPC was not statistically different between the two

populations the median value was 5 cm for both dyslexic and non-dyslexic children

Latvala et al3 failed to indicate the technique used in assessing the near point of convergence

54

but simply noted that ldquoconvergence ability was measuredrdquo but reported that convergence near

point greater than or equal to 8cm was found in 127 of the dyslexic subjects and 2 of the

control group The authors concluded that the near point of convergence was the only variable

that showed a statistically significant difference between the groups They concluded that

ophthalmic factors should not be overlooked as a contributing factor to dyslexia as they may

constitute part of the dyslexic syndrome and recommended that vision anomalies should be

corrected whenever detected

Shearerrsquos 96 findings are similar to the results reported by Latvala et al 3 He assessed the near

point of convergence using the keystone telebinoculars and reported that 12 of the

participants in his study had convergence problems

In a relatively elaborate study Evans Drasdo and Richards100 investigated accommodative and

binocular functions in dyslexia The subjects were 43 control and 39 dyslexics aged between

seven years six months and twelve years three months An intelligence quotient range of over

85 for the dyslexics and over 90 for the control with the Wechsler Intelligence Scale for

Children (Revised) (WISC-R) was recorded They measured the near point of convergence

using the royal air force (RAF) rule It was concluded that all subjects were able to report a

clear subjective break and recovery and reported no statistically significant difference between

the two groups in either the near point of convergence

Hoffman83 assessed convergence using keystone visual skills tests but grouped the results as

binocular coordination which makes it difficult to analyze and compare while in the study by

Helveston et al 85 98 of the participants had normal near point of convergence

In 2002 Keily Crewther and Crewther 12 investigated the relationship between functional

55

vision and learning to read in Australian school children Two hundred and eighty-four

children (mean age 99 18 years) received a vision screening emphasizing binocular

anomalies associated with discomfort at near (distance and near visual acuity distance vision

ldquochallenged with binocular +1 D lensesrdquo near heterophoria near point of convergence

stereopsis and accommodative facility) Children were classified as normal readers (n = 195)

children with dyslexia (n = 49) or learning disabled children (children who have learning

difficulty with impaired intelligence quotient) (n = 40) It was reported that the near point of

convergence were better in the dyslexic group compared to the normal and the learning

disabled group but found no statistically significant difference between the groups In

constrast Evans et al 91 assessed near point of convergence using the RAF rule and reported a

highly statistically significant relationship between NPC and reading retardation (p=0019)

25 Heterophoria

Heterophoria is a common binocular problem and in clinical practice it is particularly

important when analyzed in conjunction with fusional reserve as it gives an indication of the

zone of comfortable binocular vision

The distance phoria is a clinical measurement of the tendency of the two eyes to deviate from

parallelism when viewing a target usually at six meters When a closer viewing target is

utilized (say at about the individuals reading distance) the resulting near phoria measures the

degree to which the eyes over converges (esophoria) or underconverges (exophoria) to the

plane of the target97 This latent deviation requires fusional vergence to maintain single

binocular vision and at near they involve the accommodation system Smooth functioning of

these systems particularly at near is important for near tasks92

Several studies312858789959698100101 have attempted to investigate the role of heterophorias on

reading ability and all reported remarkably varying findings The assessment of heterophoria

was extensive in the study conducted by Latvala et al 3 Heterophoria at distance was evaluated

56

using the cover test and Maddox rod The prism bars was used to quantify their findings The

Maddox wing was used to assess phoria at near At near they reported a higher prevalence of

esophoria (61) among the control than the dyslexic group (58) a higher prevalence of

exophoria (25) among dyslexics than the control group (122) and a higher prevalence of

vertical phoria (61) among the control than the dyslexic subjects (36) At far distance

they found a higher incidence of esophoria (113) among dyslexics than the control (82)

subjects a higher prevalence of exophoria (2) among the control group than among the

dyslexics (19) and vertical phoria to be more prevalent in the control (41) than in the

dyslexics (38)

Keily et al 12 assessed heterophoria using the Prentice card and reported that there was a slight

but insignificant tendency for the dyslexic subjects to be more exophoric than the normal and

those with learning disability but found no statistically significant difference The test distance

was not indicated

Evans et al 100 in a study of accommodative and binocular functions in 43 control and 39

dyslexics reported that all subjects examined were orthophoric at distance with cover test

except for one dyslexic subject who demonstrated 10 prism diopters right esotropia and two

diopters right hypertropia which changed to 15 prism diopters right esotropia with two prism

dioptre right hypertropia at near At near horizontal phoria results were similar in both groups

Evans Efron and Hodge 101 studied the incidence of lateral phoria in 45 children with specific

learning disability in the United States of America in 1977 The participants were selected from

a psychologistrsquos file The control group comprised of 364 children in second third and fifth

grades attending regular school It was not indicated why there was such a big difference in

the number of participants from both groups The test used in assessing phoria was the

ldquoRocket card which was used in conjunction with red- green spectaclesrdquo They reported a

57

prevalence rate of 1135 in the control group while 222 of the children from the specific

learning difficulty group had heterophoria There was no statistically significant difference

between the groups In concluding it was recommended that phoria conditions may not cause

reading or learning disability but that the presence of such conditions could contribute to visual

perceptual and attention abnormalities

A report by Sucher and Stewart98 contained limited information as the authors did not detail

whether a higher prevalence rate among control (105) than dyslexic (86) was for near or

distance phoria Similarly Eames95 failed to indicate the distance at which the measurement

was made but reported that 33 of poor readers had exophoria and 22 of control were

exophoric but did not indicate whether distance or near

Helveston et al 85 used the Wottring amblyoscope to determine ldquoobjective and subjective angle

of ocular alignmentPrime and reported that 96 of the participants in their study were orthophoric85 The criteria used to classify ocular deviation were not indicated

Another comprehensive report of heterophoria was documented by Shearer96 The subjects

were screened using the Keystone telebinoculars and reported that 1 of the study participants

had vertical phoria (gt1 prism diopter) 2 had esophoria (gt 4 prism diopter) 3 had

exophoria (gt 4 prism diopter) 26 had exophoria at near It was concluded that vision

functions are not related to reading ability Hoffman 83 reported that ninety three (93) subjects

(869) had problems of binocular coordination but failed to indicate whether it was phoria or

tropia

Kapoula et al 87 evaluated ocular alignment using the cover test and reported that at far

distance the median value of phoria was zero for both non-dyslexic and dyslexic children (p =

019) At near the median value of phoria was similar between the two groups (minus2 prism

58

diopters for non-dyslexics and minus15 prism diopters for the dyslexics (p = 019)

Bucci et al89 measured heterophoria at near and far using the cover-uncover test As

documented by the authors exophoria was not statistically different between the two groups (p

= 02) the median was ldquo0rdquo prism diopters for dyslexics and 2 prism diopters for non-dyslexics

No information was given on other aspects of heterophoria

Simons and Grisham92 conducted a literature analysis on binocular vision anomalies and

reading emphasizing studies with positive and negative relationship between binocular vision

and reading The authors concluded that binocular conditions such as esophoria exophoria

restricted ranges of fusion foveal suppression accommodative insufficiency and oculo-motor

disorders at reading range are more prevalent among poor readers than normal readers

In another study Ygge Lennerstarnd Rydberg Wijecoon and Petterson102 investigated

oculomotor functions in a Swedish population of dyslexic and normally reading children in

1993 The study comprised 86 nine -year- old dyslexic children matched to control with regard

to age gender and intelligence Heterophoria was measured using the cover test They

reported that for the dyslexic group ldquo23 pupils exhibited a phoria (264 9 eso- and 14 exo-

phoria)rdquo whereas in the control group ldquo20 pupils had a phoria (232 6 eso- and 14 exo-

phoria)rdquo and that the difference was not statistically significant (p = 034) At near the

incidence of exophoria was 437 for the dyslexic group and 453 in the control group

There was no statistically significant difference (p=055)

26 Strabismus

Several authors34 83-85 90 93102 included the measurement of strabismus in their studies

reporting remarkably varying findings Helveston et al 85 assessed ocular alignment using the

Wottring amblyoscope and reported that only between 2 - 4 of the students had phoria or

59

tropia of more than four prism dioptres Latvala et al 3 reported the prevalence of tropia at near

for the dyslexic group to be 36 and 2 for the control group Hoffman83 assessed ocular

alignment using the keystone visual skills test and reported that 13 subjects (9 exotropes and 4

esotropes) (1214) had strabismus It is difficult to comment if this relatively high

prevalence of strabismus is related to hyperopia as the refractive error prevalence of 215 was

not categorized as to the type In a study by Sherman84 a total of four children (8) were

strabismic classified as three exotropia (alternating) and one esotropia (alternating) Rosner

and Rosner93 in their retrospective analysis reported that in the learning disabled group 51

had esotropia 39 had exotropia while in the non- learning disabled group 8 were

strabismic It was not unexpected to find a higher prevalence of strabismus in this

retrospective study Aasved 4 found that the dyslexics had a higher prevalence of manifest and

latent convergent strabismus than the control subjects

Goulandris et al90 assessed strabismus using the cover test and reported that there was no

statistically significant difference between the two groups in the prevalence of strabismus

Ygge et al 102 reported that at distance manifest strabismus was more frequent among the

dyslexic (8) pupils than the controls (23) but that the difference was not statistically

significant At near the prevalence of manifest strabismus was 8 (all eso-) for the dyslexics

and 35 (all eso-) in the control group

27 Accommodation Functions

271 Amplitude of Accommodation

The accommodation mechanism is extremely important for learning Children who suffer

some anomalies of accommodation are more prone to fatigue quickly and become inattentive

than those who have normal accommodation function According to Flax 22 an ldquoinefficient

accommodation function is a significant contributor to lowered achievement in the upper

grade

60

The findings on various aspects of accommodation functions were diverse Metsing and

Ferreira 103 studied visual deficiencies in children from mainstream and learning disabled

schools in Johannesburg South Africa in 2008 One hundred and twelve (112) children from

two learning disabled schools and eighty (80) children from a mainstream school in Johannes-

burg participated in the study The vision functions examined were visual acuity refractive

status ocular health status accommodative functions (Posture facility and amplitude)

binocularity (cover test vergence facility smooth vergences NPC and ocular motilities (direct

observation) However according to the authors only the results for accommodation

accuracy amplitude of accommodation vergence facility and saccadic accuracy are reported

since only these variables showed statistically significant relationship on the relationship

between the (mainstream and learning disabled) groups Metsing and Ferreira 103 reported

that a high percentage (516 and 533) of low amplitude of accommodation for the right

and left eyes respectively was found in the mainstream group compared to the 291 and

288 in the learning disabled group The relationships between the mainstream group and

reduced amplitudes of accommodation of the right (p=004) and left eyes (p=0001) were

found to be statistically significant (plt005) The relationship between the mainstream group

and reduced amplitude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to

be moderate and that between the mainstream group and the right eyes (Cramerrsquos V= 0286) to

be low

In the study by Grisham et al86 accommodative amplitude was assessed using the

accommodative rule ldquoThe student held an occluder over the left eye The examiner asked if

the student could see the (2030) target at the larger end of the Occlud-A Measure held at about

20 cm from the face If yes then the student was asked whether the target was in focus or ldquonot

blurryrdquo If the student responded that the target was in focus then the target was moved at

approximately 2cm per second toward the studentrsquos face and the student was asked to report

when the target first became blurryrdquo About 638 of the participants had amplitude of

accommodation of 11D which the authors considered equivalent to the expected amplitude of

61

accommodation for the age About 247 had inadequate accommodation amplitude while

115 had ldquoborderlinerdquo amplitude of accommodation

In the study by Evans et al 100 the amplitude of accommodation was measured using the push up

method They reported that the amplitude of accommodation was significantly reduced in the

dyslexic group and that the mean accommodation lag was calculated for each eye but found no

statistically significant difference between the two groups

Alvarez et al 88 assessed monocular accommodative amplitude using the minus lenses method

The subject viewed horizontal line of 2030 letters at 33cm as the examiner introduced minus

lenses in 025D increments until the target first becomes blurred According to the authors the

working distance adjustment used was kept at 250 DS to compensate for minification The

authors reported that monocular accommodative amplitude was significantly lower (p lt 0001)

in the group of poor readers (right eye 91 D 23 left eye 90 D 23) than in the control

group (right eye 105 D 17 left eye 105 D 17)

Helveston et al85 measured the amplitude of accommodation using the push up to blur

technique and reported that ldquoan ability to accommodate more than eleven dioptres was found in

between 90 and 93 of the studentsrdquo Ygge et al 102 assessed amplitude of accommodation

using the RAF rule and reported that the two groups performed similarly A similar technique

was used by Goulandris et al 90 and a lack of statistically significant difference between the

groups was reported

272 Accommodation Facility

In the same study Evans et al 100 reported that the dyslexic group appeared to be slower at a

62

test of accommodative facility The authors discredited the flipper technique as the

unnatural act of altering accommodation without changing vergence They further noted that

the flipper testhellip would still imperfectly reflect the normal visual environment owing to the

absence of a stimulus to vergence and accommodation

Grisham et al86 measured accommodative facility using the 200D lenses flipper technique

and the technique was performed as follows The 2D lenses was flipped each time the

student said the target was ldquoclearrdquo The target for this test was the 2030 target used for

accommodative amplitude The left eye was occluded The student was asked whether the

target was clear when held at about 40 cm If so then the student was told that the test would

be timed and that it should be reported whenever the target was ldquoclearrdquo The examiner then

started a stopwatch and counted the number of times the lenses were flipped in 30 seconds

That number was taken to represent the cycles per minute (cpm) and therefore to represent the

studentrsquos ability to change accommodation over time Based on the criteria used in their study

the authors reported that 76 of the students had adequate monocular accommodative facility

while 236 had ldquoweak accommodative facility of 9 cpm or less and 81 had accommodative

facility of as low as 1 or 0 cpm

In the study conducted by Alvarez et al 88 accommodative facility testing was conducted using

the following procedure The subject wearing his or her habitual correction was seated and

positioned so that the spectacle plane was 40cm from the Bernell Acuity Suppression Slide

(VO9) A 200 D accommodative demand was used with a 2030 test target at 40cm The

patient was asked to view the reduced Snellenrsquos letters 2030 in line 5 and say ldquonowrdquo when the

letters in that line appear clear and single and to report letters missing from lines 4 and 6

during the binocular testing They reported that the binocular accommodative facility values

were significantly lower (p lt 005) in the poor readers group (49 cpm 31) than the control

group (63 cpm 29)

63

Sucher and Stewart 98 assessed binocular accommodative facility at 35 cm using 150 diopter

flippers lens for ldquoone minuterdquo and ldquoused the University of California Berkeley clinic standard

of 20 cycles90s as a ldquocriterion for successrdquo found a higher prevalence of accommodative

infacility in dyslexics (222) than the control group (86)

On the contrary three authors6 12 93 found the control subjects to have better accommodation

facility than the dyslexic subjects The studies are Rosner and Rosner93 found a higher

prevalence of accommodative infacility among the control (10) than the learning disabled

group (9) Similarly Buzzelli6 found that the dyslexic subjects showed better

accommodation facility than the control group To assess accommodation facility ldquopatients

were asked to clear the number 9 on the Bernell Vectogramm Target at 40cm hellip alternately

viewed the target through a + 200DS and a ndash 200 DS lenshellip for 20 cycles) There was no

statistically significant relationship between the two groups

Keily et al12 assessed accommodative facility using +200DS and ndash 200DS flippers lenses

with an N6 fixation target for one minute at 30 cm rather than 40 cm because according to the

authors the children were required to hold the reading material They reported that

accommodative facility was better in normal readers than either of the poor reading groups

In another study Evans and Drasdo104 reviewed ophthalmic factors in dyslexia and commented

that convergence insufficiency accommodative dysfunctions and unstable eye movements are

more common in dyslexic population than in normal readers

Latvala3 simply noted that some aspects of accommodation were difficult to measure and failed

to give any details

64

273 Relative Accommodation

Alvarez and Puell88 measured relative accommodation using a phoropter The subject viewed a

horizontal line of 2030 letters at 40cm with hisher best refractive correction The examiner

introduced minus or plus spherical lenses in 025 D steps binocularly until the patient reported

first sustained blur Negative and positive relative accommodation values were similar in both

groups This was the only study accessed that reported on relative accommodation

274 Accommodation Posture

Evans et al100 assessed accommodation lag using the monocular estimate method of retinosocpy

and reported no statistically significant difference between the dyslexic and the control groups

Metsing and Ferreira103 measured the accuracy of the accommodative response (lag or lead)

objectively using the MEM retinoscopy in normal room illumination and found that the subjects

from the mainstream school demonstrated a high prevalence of lead of accommodation (40

and 405 for the right and left eyes respectively) However in the learning disabled group a

high prevalence of lag of accommodation (98 and 119 for the right and left eyes

respectively) was found The relationships were found to be statistically significant (p lt 005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000)

Sherman84 assessed accommodation amplitude and relative accommodation using minus lenses

and accommodative facility using the flipper techniques The lens powers for the plus and

minus lenses used to assess accommodation functions were not indicated His findings may be

difficult to utilize due to the somewhat unique way of classifying the results as ldquomechanical

problemsrdquo

Flax22 suggested further investigations in the nature of accommodative function in reading

65

disabled children indicating that the use of the push-up method of assessing the amplitude of

accommodation is unreliable due to its lack of sensitivity Flax22 reiterated that

accommodation has a contribution in learning disabilities and that accommodative function

inefficiencies tend to play an increasing role as the youngster moves through school and that

inefficient accommodation function is a significant contributor to lowered achievement in the

upper grade

28 Vergence Facility

Vergences are a clinical measure of an individualrsquos ability to overcome the prismatically

induced optical displacements of a target in space Convergence relates to each eye fixating

closer in space than the real physical location of the target while divergence is the measure of

the eyes to fixate further in space while still maintaining single binocular vision97

Buzzelli 6 assessed vergence facility as follows ldquopatients were asked to align a vertical picture

(box-X-O) slide made of anaglyph material for monocular presentation under binocular

conditions The target measured 21 mm in the vertical meridian and 10mm in the horizontal

meridian The subjects alternately viewed the target through 16 prism base out and 4 prism

base in The test was continued for 20 cyclesrdquo The author reported that the dyslexics

completed the vergence eye movement task significantly slower (300 seconds) than the normal

readers (240 seconds) A statistically significant relationship between the groups was reported

(p lt 005) It was recommended that the possible role of vergence in dyslexia should be

investigated on a larger sample of dyslexic and control groups

Metsing and Ferreira103 assessed vergence facility test at near (40 cm) with the subjects

presented with a vertical row of letters (69) using the 8 PD (BI) and 8 PD (BO) mounted in a

flipper device They reported that the prevalence of poor vergence facility (205) was found

to be higher in the mainstream group compared to the learning disabled group (183) The

relationship between the mainstream group and poor vergence facility (p = 0000) was found

66

to be statistically significant (plt005) A medium to strong relationship was found to exist

between the mainstream group and poor vergence facility (Cramerrsquos V = 0369) It was

concluded that children from mainstream schools are likely to present with poor vergence

facility compared to children from schools of the learning disabled

2 9 Fusional Reserves

The compensatory effect of fusional vergence reserves on phoria is very important as difficulty

with reading is likely to occur when there is uncompensated phoria in conjunction with

reduced fusional vergence reserves 92

A major study was reported by Evans et al 100 which found that negative and positive reserves

were significantly reduced in the dyslexic group relative to the control The ldquoprism vergence

amplitude calculated as the difference between base out and base in break and recovery points

was relatively reduced in the dyslexic grouprdquo They remarked however that several subjects

were unable to appreciate a subjective blur point but the results for the break and recovery

were complete

In the study conducted by Latvala et al 3 the incidence of fusional amplitude greater than or

equal to thirty two prism dioptre at near was higher in the dyslexic (75) than in the control

(61) group At distance they found fusional amplitude greater than or equal to15 prism

dioptre to be higher in the control (122) than in the dyslexic (94) group

Disordered vergence control was studied by Stein Riddell and Fowler 105 in the Uniteed

Kingdom in 1988 The subjects were thirty nine dyslexic children aged eight to eleven years

and a control group of twenty subjects of similar age group The vergence control was

measured using the Synoptophore They reported that two thirds of the dyslexic group had

67

abnormal vergence control According to the authors vergence eye movements never seem to

have been investigated in detail during reading before and that the study was the first in which

eye movement recording have been used to demonstrate differences between dyslexics and

normal readers

According to Grisham et al86 ldquoConvergence and divergence break and recoveryrdquo were

measured at 40 cm with a horizontal prism bar The student was first asked if the clown face

target on the smaller end of a standard optometric measure (Occlud-A-Measure Bernell

Mishawaka Indiana) could be detected If so then the student was requested to say whether

the target appeared single or double Once single vision was obtained the bar was moved

relative to the studentrsquos eye until double images were reported That point was considered

ldquobreakrdquo Then the motion of the bar was reversed until the student reported the image single

and this point was considered ldquorecoveryrdquo This procedure was first performed for base in (BI)

position of the bar over the left eye and repeated with base out (BO) position over the right

eye The authors reported that a large number of students had poor convergence skills 38

break at less than 18 PD and 95 recover at 7 PD or less These values were considered to be

in the ldquoweakrdquo to ldquovery weakrdquo range and remarked that in contrast to convergence lsquomodalrdquo

values for break (14 PD) and recovery (10 PD) were not in the ldquoadequaterdquo range but would be

classified as ldquovery weakrdquo for break and ldquoweakrdquo for recovery As a result very high

percentages of students had poor divergence skills 82 break at less than 20 PD and 60

recover at less than 11 PD

Kapoula et al 87 measured vergence reserves using prism bars at far and near distance for both

groups They reported that the median value for positive fusional vergence was 16 prism

diopters and 14 prism diopters for non-dyslexics and dyslexics respectively at far distance (p

= 012) and 20 prism diopters for both non-dyslexics and dyslexics at near distance For

negative fusional vergence they reported that ldquodivergence amplituderdquo was significantly

different in the two groups for both distances (p lt 0005) At far the median value was 6

68

prism diopters and 4prism diopters in non-dyslexics and dyslexics respectively and 12 prism

diopters and 10 prism diopters respectively at near

Bucci et al 89 measured fusional reserves at far and near using prism bars (base in and base

out) According to the authors orthoptic evaluation of vergence fusion capability showed a

significantly limited divergence capability for dyslexics compared to non-dyslexics the

median value was 10 prism diopters for dyslexics versus 17 prism diopters for non-dyslexics

In contrast convergence amplitude was in the normal range for the two populations while for

dyslexics the amplitude was significantly larger than for non-dyslexics the median value was

30 prism diopters and 18 prism diopters for dyslexic and non-dyslexic children respectively

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups

The mean fusional convergence capacities at distance were 1680 prism diopters for both the

control and the dyslexic group At near the corresponding figures were 2640 prism diopters

and 267 prism diopters respectively The mean fusional divergence capacity at distance was

650 and 620 prism dopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 105 and 102 prism diopters respectively There was no

statistically significant difference between the groups

210 Ocular Pathology

Ocular pathologies do not seem to be prevalent in the dyslexic population and it was not

reported on in the literature reviewed However according to Evans and Drasdo100 the

perspective to relate ocular pathology to dyslexia is in individuals who were normal readers but

developed reading disability due to pathological conditions such as stroke or tumors affecting

the right hemisphere These conditions may be relevant to the considerations of establishing the

relationship between ophthalmic problems and dyslexia100

69

Only one study83 included this as part of the investigation Hoffman83 reported that one child

(093) of the population of learning disabled children examined had acute conjunctivitis

A major study on dyslexic subjects (although outside the cope of the present study but worth

citing) was conducted by Raju11 in 1997 She studied oculomotor control in dyslexia and

reported that sixty five percent of the normal readers exhibited normal oculomotor control as

compared to the eleven percent of dyslexics and that fifty four percent of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills whilst twelve

percent of the control subjects displayed the same

70

LITERATURE REVIEW TABLE pages 70-72

71

72

73

211 Summary

Research into visual aspects of dyslexia has been undertaken mainly by optometrists and

ophthalmologists The main area of controversy is that ophthalmologists tend to deny any

relationship between vision and reading ability while optometrists assert that identifying and

correcting visual defects are an important part of managing dyslexia The findings of studies that

have investigated vision function in dyslexic population since the 1940s are inconclusive and

reflect the complexities associated with identifying the causes of dyslexic as well as its diagnosis

treatment and management

A range of study designs were used and the inclusion of a control group for example depended

on the objective of the study Some authors either compared the prevalence of vision defects in a

population of reading disabled group to the prevalence of similar vision defects in a group of

normal readers or only presented the prevalence of a particular visual condition in a group of

reading disabled children All the studies used convenience sampling methods at schools or by

including learners who were referred to their private practices the size being determined by the

number of dyslexic students available The main areas of vision functions studied were visual

acuity refraction binocular vision and ocular pathology The study findings were inconclusive

and the only vision variables consistently report ed across the studies was the association of

hyperopia with poor reading performance

The differences in results reported by the authors may be due to the methodological problems

such as differences in instrumentation and techniques failurecut-off criteria method of data

analysis lack of comparison group and subjectsrsquo selection from clinical population There was a

noticeable absence of studies on African children research has shown that African persons are

less at risk of myopia and more at risk for open-angle glaucoma than Caucasians There is

therefore scope for continued research among African children to determine whether there is any

relationship between dyslexia and vision

Research on dyslexia and vision that requires an interdisciplinary approach may yield different

results due to different approaches by different professionals However it is challenging to

74

design a study that would control for all variables possibly influencing reading ability and vision

function Reading performance is an extremely complex task and its measurement can be

influenced by many factors

Having reviewed the relevant literature on dyslexia and vision Chapter three will present the

research design the study population sampling procedure methods of data collection and

analysis

75

CHAPTER THREE METHODOLOGY

31 Introduction

This chapter will present the research design the study population sampling procedure

methods of data collection as well as method of data analysis

32 Research Design

The study was designed to provide empirical information to enable a comparison of the visual

characteristics of dyslexic children and normal readers A matched paired case-control

method was adopted Data was collected from both the dyslexic and control groups and

analysed

321 Sampling Design

Participants for both samples were selected using the convenient sampling method (based on

available subjects) and an optometric eye examination was conducted

322 Study Population and Participants

The study population comprised children attending a school for children with learning

difficulties from which the dyslexic children were selected to participate in the study while

the control group consisted of learners from a mainstream Durban school

i Dyslexic Group Participants for the study from the dyslexic group consisted of 31

African dyslexic children (the experimental group) selected from Khulangolwazi Special

School for children with learning difficulties in Clairwood South of Durban Psycho-

educational evaluationdiagnosis of dyslexia was not part of this study and was not

76

performed The school principal allowed access to all learnersrsquo file from which the

dyslexic learners where selected based on the school psychologistrsquos diagnosis Learners

attending this special school were referred from mainsream schools around Durban The

final diagnosis as to the type of learning difficulties was done by the school psychologist

None of the children were on any medication

The mean age for the dyslexic participants was 13 years and the learners were in grades

four through grade seven considering the fact that they were children that were reading two

grades behind their chronological age There were 15 boys and 16 girls Due to limited

number of learners who were classified as dyslexic it was difficult to recruit the targeted

number of one hundred participants for the study The subjects from both groups were

lsquoAfricanrsquo South Africans of African origin

ii Control Group Participants for the control group consisted of 31 children from a

mainstream school in Durban (Addington Primary School) The mean age of the children

was 11 years and 9 months A total of 15 boys and 16 girls were chosen ranging from grade

four to grade seven

323 Inclusion Criteria for the Dyslexic group

To be included in the study the subjects had to meet the following criteria

1 An average or above average intelligence quotient For this study an average was taken

as between 95 and above as recorded on the learners file by psychologist

2 Be two grades or more below grade equivalent in a mainstream school

3 The child has not been absent from school for more than 10 of the attendance days

The information on the learnersrsquo attendance was supplied by the school principal

324 Exclusion Criteria for the Dyslexic group

The following criteria resulted in children being excluded from the study

1 Presence of any emotional problems (information from the learnersrsquo file)

2 Presence of any systemic condition

77

3 Use of any systemic medication

The inclusion and exclusion criteria for the control group were similar to the experimental

group except that the children did not have any reading problems and were attending a

mainstream school

33 Ethical Clearance

The study protocol was approved by the then University of Durban-Westville research ethics

committee A letter of request for access to the schools was sent to the Department of

Education (Appendix B) Written informed consent (Appendix C) for access to the schools (for

both groups) was obtained from the Department of Education Written informed consents were

also obtained from the principals of the schools (mainstream and special school) (Appendix D)

Due to the difficulty in reaching the parents the school principals consented on behalf of the

parents for the learners to participate in the study The participants were fully informed of the

purpose of the study and accepted to co-operate with eye examination procedures

34 Testing Protocol

The testing was done first with the dyslexic learners and then at the mainstream school

35 Study Setting

The same testing arrangement was used for both groups A testing room was set up in one of

the offices provided by the school principal The distance acuity chart was placed at a point

six meters away from where the subjects sat and was used for the distance acuity chart and all

distance testing The examinations were done under room illumination except for some

procedures such as retinoscopy that required dim illumination resulting in the room being

dimmed

78

All testing was conducted in the morning as it was anticipated that better responses could be

obtained when the children were not tired Each school principal provided an assistant who

helped with various activities such as controlling room illumination The rationale and

technique for every procedure was fully explained to each participant and a trial reading was

taken to ensure that all instructions given were understood During the tests subjects were

asked (from time to time) if they were tired For any answer to the affirmative the testing was

discontinued and the child was allowed to have a break As a routine a break of 10 minutes

throughout the duration of entire procedure was allowed

The testing instruments were provided by the International Center for Eye Care Education

(ICEE) As the instruments were also being used by other researchers they were unavailable

on certain days resulting in some tests being postponed until a later date The data collection

took an average of two months per school All data were collected by the examiner alone and

each examination took an average of thirty minutes to complete

36 Data Collection Techniques

All tests were conducted in free space The data collection techniques used in the present

study was similar to techniques used in other studies 93100106-140 based on pediatric

populations (Table 31)

Table 31 Data Collection Tools and Measurement Outcome

InstrumentationTechnique Measurement Outcome

Bailey ndashLovie LogMAR Charts Visual Acuity

Royal Air Force (RAF) Rule NPC Amp of Accommodation

Welch Allyn Streak Retinoscope Objective Refraction

Trial Frame and Trial lenses Subjective Refraction

Cover Test - Occluder and Prism Bar Ocular Alignment

Maddox Rodpen torchPrism bar Heterophoria and Tropia

Maddox Wing Near Phoria

Prism Bar Heterophoria and Tropia

79

Pen torch PD rule Cornea reflex test-strabismus

Retinoscope and neutralizing lenses Accommodation Posture

Jackson cross cylinder Refine subjective findings

Trial Frame and neutralizing lenses Relative accommodation

+-2 flipper lenses Accommodation Facility

Direct Ophthalmoscope (Welch Allyn) Ocular Health

Prism Bars Fusional Reserves

361 Visual Acuity

Visual acuity is a measure of the acuteness or clearness of vision It assesses the ability of the

visual system to resolve detail which is dependent on the sharpness of the retinal focus within the

eye the sensitivity of the nervous elements and the interpretative faculty of the brain 106 Visual

acuity was assessed using the Logrithm of Minimun Angle of Resolution (LogMar) chart which

means that the letters change in size from line to line in equal steps of the log of the minimum

angle of resolution (MAR) Visual acuity was assessed monocularly and binocularly for both

distance and near The LogMar chart facilitates algebraic operations for statistical analysis and

allows for precise quantitative assessment of visual acuity It is recommended for use in research

studies in which visual acuity is a dependent variable106

Each subject was comfortably seated and the test was conducted with the examiner sitting in front

in such a way that the subjectrsquos view was not obstructed For monocular visual acuity the

subject was instructed to cover one eye with their right palm and to read the letters on the chart

This procedure was repeated binocularly and the test was done under normal room illumination 106 No subjects wore spectacles so visual acuity for aided vision was not indicated

362 Refractive Error

Refractive error was assessed objectively using the Streak Retinoscope (Welch Allyn) with a

+150D fogging lens (for an armrsquos length of approximately 67cm) with the subject fixating a

660 (to maintain fixation) optotype on the distance visual acuity chart As the subject focused

80

on the distance target on the chart the retinoscope light was passed across the subjects eye

(right hand used to scope right eye and left hand used to scope left eye) The motions

observed were neutralized with the appropriate lenses plus lenses for lsquowith motion and minus

lenses for against motion The subjective refraction was performed through monocular

fogging refined using the Jacksonrsquos cross cylinder followed by binocular balancing 106 The

retinoscopic results were used for analysis

Although retinoscopy was performed without cycloplegia as in other studies 395107108

cycloplegic refraction was not performed because the researcher (an optometrist) was not

licensed to use therapeutic drugs when the data was collected However it has to be

acknowledged that a non-cycloplegic examination among children has limitations among

those with hyperopia as the full extent of the hyperopia may be masked

363 Near Point of Convergence

The NPC was measured using the Royal Air force (RAF) rule The subject was instructed as

follows ldquoThis test measures your ability to turn your eyes in towards your nose Look

directly at the dot on middle of the line The image may appear to be blurring (not clear)

That is okay However if the dot becomes two say lsquotworsquo I will then pull the target away If

the dot becomes one again say lsquoonersquo rdquo The subject was asked to fixate on the dot on the

middle of the line as the target was advanced towards him The objective reading was taken at

the point when one eye loses fixation while the subjective reading was noted when the subject

reported the target to be double At a point when the subject reported double the target was

moved back until a point when the target was reported to be single again this was taken as the

recovery point The final break and recovery points recorded were the average of three tests

measurements The measurement was taken three times in order to detect fatigue which may

indicate poor convergence 111-113 The objective reading was taken for analysis

364 Strabismus

Strabismus was assessed using the Hirschberg test A penlight was held about 50cm from the

childs face in the mid plane and they were encouraged to fixate the light with both eyes open

81

The location of the corneal reflection on each cornea relative to the centre of the pupil was

then noted The displacement of the corneal reflection from the centre of the pupil (angle

lambda) was estimated in millimeters An ocular alignment was observed as a symmetric

displacement of the reflexes 114115 The examiner proceeded with the Kappa Test which is a

test of monocular fixation and is done while the child was monocularly viewing the examiners

penlight The penlight was maintained at 50 cm distance from the face The purpose of this

test was to determine the visual axis of each eye The examiner observed the location of the

corneal reflexes relative to the center of the pupil in the right eye and then in the left eye A

nasal displacement is signified by a (+) sign and a temporal displacement is signified by a (-)

sign with the amount quantified in millimeters away from center Again a normal location

will likely be either centered or +05 mm nasally displaced 114115

The interpretation of the magnitude and the type of deviation requires the data gathered from

both the Hirschberg tests The fixating eye is the eye in which the corneal reflex location is

the same on the Hirschberg and Kappa tests The strabismic eye is the eye in which there was

a difference The change in location from the Kappa tests as compared to the Hirschberg test

equals the amount and type of deviation Every 10 mm of corneal displacement is

approximately equivalent to 22 prism diopters A nasal change in location on Hirschberg

testing from the visual axis denoted by the Kappa test denotes an exotropia A temporal

change in location on Hirschberg testing from the visual axis denoted by the Kappa test

denotes an esotropia 114

365 Heterophoria

Ocular alignment was assessed using the cover test at six meters and 40cm for distance and

near respectively It is an objective technique and is very suitable for assessing ocular

alignment in children

For distance cover test the test target was a letter from the line above the subjectrsquos best visual

acuity of the worst eye The subject was advised that the muscle balance of their eyes was

assessed and that he should look steadily at the fixation target (an isolated letter on the chart)

at all times and ignore the occluder The coveruncover (unilateral) cover test was performed

82

to determine the presence of phoria or tropia 106114-115 To perform the unilateral cover test

the occluder was placed in front of the right eye held for approximately one to two seconds

and the left eye was observed for any movement If no tropia or phoria present the left eye

was covered and the right eye then observed Any movement observed was neutralized using

a loose prism bar in the corresponding base direction Base-in prism for exo deviation and

base-out for eso deviation

The alternating cover test was performed to assess the direction and magnitude of phoria or

tropia (it measures the total angle of deviation) 106114-115 The test conditions were the same as

the unilateral cover test The subject was comfortably seated and instructed to look at the

fixation target at distance The occluder was placed in front of the patientrsquos right eye held for

2-3 seconds and then quickly placed on the left eye The test was repeated several times with

the occluder held in front of each eye before quickly moving to the other eye while observing

the eye that was just uncovered Any deviation was neutralized accordingly 115 The Maddox

Wing was also used to assess near phoria under normal room illumination The technique was

assessed with the subject wearing his full subjective correction 115 The use of cover test for

the assessment of ocular alignment at both distance and near may have allowed for continuity

and standardization However the cover test and the Maddox wing were used by Evans et al 100 in the same study The subject was directed to look through the horizontal slits to view the

chart that comprised of a horizontal and vertical scale and a horizontal and vertical arrow The

construct of the Maddox wing is such that the right eye sees only the arrows while the left eye

sees only the scale It achieves dissociation by presenting independent objects to the patient

As the images are dissimilar the incentive to fusion is abolished and the eyes adopt the fusion

free position 115 The arrows are positioned at zero on the scales but through dissociation any

phoria will be indicated by an apparent movement of the arrow along the scale

To measure horizontal phoria the subject was asked ldquowhich white number does the white

arrow point tordquo The number on the scale represents the magnitude of the deviation and the

direction To measure vertical phoria the subject was asked to indicate which red number the

red arrow points to 115

83

366 Evaluation of Accommodation Functions

a Accommodation Posture The assessment of accommodation accuracy was performed at

the subjects habitual near distance using the monocular estimation method (MEM) The

MEM technique uses a specially made target with pictures or words suitable to the childs

reading level The card was attached to the retinoscope The procedure was performed

over subjects distance prescription and at his normal reading distance with enough light

to provide a comfortable reading vision Retinoscopy was performed on axis (right eye

before left eye) as the child reads the words or describes the picture on the card aloud As

the retinoscopic reflexes were observed an estimate of the magnitude of the motion was

made and trial lenses were then interposed briefly (approximately 05 seconds) to avoid

changing the childs accommodation status until no motion was observed The lens

power at which the motion was neutralized was taken as the lag or lead of

accommodation It is a lag of accommodation when the reflex is neutralized with

positive lenses while it is called lead of accommodation if neutralized with negative

lenses 112113116117

b Accommodation Facility this assesses the rate at which accommodation can be

stimulated and relaxed repeatedly during a specific time period It relates to the

individualrsquos ability to shift focus quickly and efficiently for varying distance The

procedure was performed with the best subjective correction in place monocularly and

binocularly but only the result for the binocular assessment was utilized for the data

analysis According to Wick and Hall 120 binocular accommodative facility testing may

not be a true accommodative facility measure such as monocular facility but clinically

binocular testing may be more useful because it gives a ldquoreal liferdquo assessment of

accommodation and binocular interactions of accommodation and vergence

The target used was letters on a 69 range on a near point card and at the subjectrsquos

habitual reading distance A lens flipper of 2 diopter lenses was used Letters and

pictures used were appropriate to the childs ability The subject was informed that he

would be shown two different lenses and that one pair causes the system to work while

84

the other pair relaxes it and was instructed to look at the materials as the lenses were

flipped Each time the print become clear and single he was to immediately report

clear The lenses were flipped and child should report clear when the material gets

clear To ensure that subject understood the instructions trial readings were first taken

The number of cycles completed per minute was recorded One cycle meaning clearing

both the plus and minus lens sides The test was done binocularly112-113118-120 At near

with no suppression control

c Amplitude of Accommodation The amplitude of accommodation was assessed by

Donderrsquos push ndash up method using a Royal Airforce (RAF) near point rule a rod with a

movable target and metrics as well as dioptric marking This procedure was performed

monocular and binocularly The subject was seated comfortably and instructed to read a

line of letters on the card that corresponded to a visual acuity of 69 and told to keep the

letters clear The target was slowly moved towards the child along the rule until the child

reported first sustained blur at which point the dioptric result was read off the Royal air

force rule Three readings were performed and an average taken This test was

conducted monocularly for each eye and then binocularly

As reported by Wick and Hall 120 the minimum amplitude of accommodation expected is

based on the subjects age and can be estimated from the Hofsetterrsquos formula

Minimum =15 - 025 (age)

Expected =185 - 03 (age)

Maximum = 25 - 04 (age)

The amplitude of accommodation was generally considered clinically important only

when it falls below the expected age minimum 120

d Relative Accommodation (RA) Relative accommodation tests are plus-to-blur (negative

relative accommodation) (NRA) and the minus-to-blur (positive relative accommodation)

(PRA) The relative accommodation tests assess patientsrsquo ability to increase and decrease

accommodation under binocular conditions when the total convergence demand is constant 106 It is also an indirect assessment of the vergence system since the vergence demand

85

remains constant while accommodative demand varies 121122

Relative accommodation was measured using the plus and minus lenses binocularly at

near with distance subjective results in place 122 In this procedure the child viewed a

reading target size letter size or picture of 66 size at the reading distance with the best

subjective correction in place Plus or minus lense powers was added in 025 diopter

steps in approximately 2 seconds interval121 to assess negative and positive

accommodation respectively This test was performed binocularly and the reading was

taken when the subject reported a sustained blur The relative accommodation is difficult

to assess with trial lenses121 The trial lenses were used to assess relative accommodation

in the present study The use of the phoropter was not possible considering the study

setting

367 Fusional Reserves

The term fusional reserve is used synonymously with vergence reserves and prism vergence

vergence amplitude 121123 or fusional amplitudes 111 Vergence ability is measured using

prisms placed in front of each eye The amount of prism is increased gradually to measure the

amount of fusional reserve the individual has in reserve to compensate for a phoria112 124 The

amount of base out prism required to produce diplopia is called positive fusional reserves or

positive fusional vergence while the amount of base in prism required to produce diplopia is

called the negative fusional reserves or negative fusional vergence 115

Positive fusional vergence (convergence) and negative fusional vergence (divergence) were

assessed using a prism bar in steps without suppression control although it has been reported

by Wesson et al 125 that when suppression is controlled the average vergence values will be

lower because the test is stopped when the suppression is detected that is if suppression is not

monitored the break is not detected until the stimulus is outside the suppression zone and a

higher vergence value may be obtained However

Scheiman et al 126 argued that the significance of such suppression in binocular individuals is

unknown

86

Assessing fusional reserves in children with loose prism bars may be a better technique

because it offers the possibility of viewing the eye movement objectively Secondly it

removes the restriction imposed by testing children behind the phoropter Finally the test

itself nearly duplicates the movement of the eyes under normal conditions allowing

convergence or divergence in discrete steps rather than as a smooth-pursuit type vergence

movement 125

The test was performed with the target at six meters (test target 69 letter line) and at 40 cm

(N5 text) The prism was placed in front of the right eye first Base-in was routinely assessed

first the rational for this sequence of testing being that the convergence response stimulated

during the base-out (or convergence) measurements may produce vergence adaptation ( a

fusional after-effect) which may temporarily bias the subsequent base in values in the base-out

direction 127 Typically there is no blur point for base -in vergence testing at distance This is

because at distance accommodation is already at a minimum and cannot relax beyond this

point 115

Base out prisms were used to measure positive fusional vergence while base in prisms were

used to measure negative vergence The subject was seated comfortably and instructed as

follows ldquoI am measuring the ability of your ayes to converge and diverge Watch the row of

letters Tell me when they first get blurry if they do Also tell me when they first become

two rows of letters and when they become one againPrime The prism was introduced over the

right eye lsquoDoublersquo was first demonstrated to the child by placing a 20 diopter prism over one

eye If blur was reported the prism power was increased until a break was reported The

prism power was then reduced until the subject reported that the rows of letters were single

The technique was then repeated for base out prism and then the test was repeated at near The

break and recovery points were determined subjectively from the childrsquos report of blur break

and recovery and objectively by observing the subjects eye movements The objective

findings were used for analysis

87

368 Ocular Health

The only test performed to evaluate ocular health was direct ophthalmoscopy

369 Vergence Facility

It was impossible to perform the vergence facility testing as the children could not fuse the

base in prisms So vergence facility data was not available for analysis The recovery point

of the base in reserves may provide useful information on the vergence facility According to

Bishop 111 ldquoblur and break points tend to reflect the quantity of fusion whereas the recovery

point indicates the quality of fusion that is the ease of change of fusional demands (facility)

and the ability to maintain fusion (stamina)

37 Statistical Analysis

The data entry was done by a staff optometrist who is skilled in statistical analysis The data

analysis was undertaken by the International Center for Eye Care Education statistician Data

was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 For comparison all data from both groups was subjected to a two-sample t-test (2-

tailed) (Table 32) Participants who did not meet the pass criteria for any of the test variables

were referred to their optometrist for further evaluation

Table 32 The Diagnostic Criteria for Each Test Variable

Variable Diagnostic Criteria

Visual Acuity 128-131 69

88

Refractive Errors 128-131

Hyperopia ge +075

Myopia ge -050

Astigmatism ge -075

Anisometropia ge 075 between the two eyes

Emmetropia lt - 050DS lt +100DS lt -075DCyl

Near Point of Convergence 132-134 ge 10cm

Accuracy of Accommodation 116117119120

Lag ge +075 Lead any minus finding

Accommodative Infacility 118119 gt 7cycles per minute in response to +2 -200 flipper testing

binocularly at near

Amplitude of Accommodation106119120

Binocular accommodative amplitude ge 2D below the expected

value for the patients age for minimum amplitude (using

Hofsetterrsquos formula) (15-14age)

Positive Relative Accommodation 106121122124

gt - 237DS

Negative Relative Accommodation 106121122124

gt + 2 DS

Phoria 106128

Distance

Near

gt 6 prism diopters exophoria or 4 prism diopters esophoria

gt 6 prism diopters exophoria or 4 prism diopters esophoria

Strabismus 131135 Manifest or intermittent manifest deviation of gt 2 prism

dioptres 135 or asymmetry in Purkinje reflex or compensating

fixation a misalignment of ge 25 degrees 131

Fusional Reserves LimitsDistance 119

Near 106121126

Base in Blur X Break 6 Recovery 4

Base out Blur 10 Break 16 Recovery 10

Base in Blur 14 4 Break12 5 Recovery 7 4

Base out Blur 22 8 Break 23 8 Recovery 16 6

89

38 SUMMARY

The study consisted of two groups of 31 participants from two schools in Durban The study

group was children with learning difficulties from which the dyslexic children were selected

and the control group was selected from a mainstream school The mean age for the dyslexic

participants were 13 years while the mean age of the children from the control group was 11

years and 9 months A convenience sampling method was adopted to select the study

participants due to the limited number of subjects to select from

The vision functions investigated may be divided into three broad areas of visual acuity and

refraction binocular vision and ocular pathology which corresponds with areas of vision

function investigated in the literature review Standard optometric testing procedures were

used to assess the vision variables

Visual acuity was assessed using the LogMAR chart at both distance and near Refractive

error was determined objectively using a streak retinoscope (without cycloplegia) while the

child fixated a 660 optotype at 6 meters with a fogging lens to control accommodation

The near point of convergence was measured using the Royal Air force rule (RAF) Ocular

alignment was assessed using the cover test at six meters and 40cm for distance and near

respectively For distance cover test the test target was a letter from the line above the

subjectrsquos best visual acuity of the worse eye The coveruncover (unilateral) cover test was

performed to determine the presence of a phoria or tropia The alternating cover test was

performed to assess the direction and magnitude of the phoria or tropia The test conditions

were the same as the unilateral cover test The Maddox Wing was also used to assess near

phoria under normal room illumination Strabismus was assessed using the Hirschberg test

with a penlight held about 50cm from the childs face in the mid plane and was encouraged to

fixate the light with both eyes open The location of the corneal reflection on each cornea

relative to the centre of the pupil was then noted

The different aspects of accommodation functions were also measured The assessment of

accommodation accuracy was performed at the subjects habitual near distance using the

90

monocular estimation method (MEM) The accommodation facility was assessed using a 2

diopter lens flipper with the best subjective correction in place The target used was letters on

a 69 range on a near point card and at the subjectrsquos habitual reading distance The amplitude

of accommodation was assessed by Donderrsquos push ndash up method using a Royal Airforce (RAF)

near point rule This procedure was performed monocular and binocularly with a 69 visual

acuity as the test target The relative accommodation was measured using the plus and minus

lenses The test target is a reading letter target size or picture of 66 size at the reading distance

with best the subjective correction in place This test was performed binocularly and the

reading was taken when the subject reported a sustained blur

The fusional reserves were assessed using prism bars in steps without suppression control

The test was performed at 6 meters (test target 69 letter line) and at 40 cm (N5 text) Base out

prisms were used to measure positive fusional vergence while base in prisms were used to

measure negative vergence Ocular health evaluation was assessed using the direct

ophthalmoscope

Data was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 To compare the data from both groups all data was subjected to a two sample t-test

(2-tailed) The methods used in this study were the same as those used for similar tests in the

international literature reviewed in Chapter Two and the sample size was larger than most of

the studies presented

Chapter Four will outline the study results with the comparative and descriptive statistical

analysis

91

CHAPTER FOUR RESULTS

41 Introduction

This chapter presents the study findings and the analysis of results obtained The data for the

prevalence of visual acuity refractive errors near point of convergence accommodation

functions and heterophoria are presented in histogram while the descriptive statistics are

presented in tables

42 Prevalence of Vision Defects

The prevalence is an indication of the subjects that have either achieved or did not achieve the

pass fail criteria and is not a normal distribution of a particular condition The prevalence rate

of the vision functions is expressed in percentages The other data is expressed as mean (M)

and standard deviation (SD) and calculated for descriptive purposes at 95 confidence

interval (CI)

If an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants would be expected

421 Visual Acuity (VA)

i Dyslexic Group In total 32 of subjects in the dyslexic group had visual acuity

less than 69 while 68 had visual acuity of 66 and 65 The distribution of those

subjects with VA le 69 is as follows 65 (2 subjects) had VAs of 69 65 (2

subjects) had VA of 612 65 (2 subjects) had VA of 615 3 (1 subject) had

VA of 630 3 (1 subject) each had VA of 638 648 660 respectively (Fig

41)

ii Control Group In total 32 of subjects in the control group had visual acuity

92

less than 69 while 68 had visual acuity of 66 The distribution is as follows 3

(1 subject) each had VA of 69 612 615 624 while 65 (2 subjects) had VA

of 630 638 and 648 respectively (Fig 41)

Fig 41 Prevalence of Visual Acuity

The mean visual acuity for the right eye for the dyslexic group was 017 031 and 000

024 for the right eye for the control group There was no statistically significant difference

between the two groups (p = 029) (Table 41) The mean visual acuity for the left eye for the

dyslexic group was 020 033 and 000 024 for the control group There was no

statistically significant difference between the groups (p = 023) (Table 41) LogMAR acuity

of 020 is equivalent to 69 in Snellenrsquos notation (Appendix E Visual acuity conversion table)

Table 41 Descriptive Statistics for Visual Acuity Variable Group N ( 95 CI) Minimum Maximum p

Mean (LogMAR)

SD

VAcuityRE

LE

DyslexicControl

DyslexicControl

3131

3131

017000

020000

031024

033024

000000

000000

010090

100090

029

023

422 Refractive Error

i Dyslexic Group Twenty three percent (23) of the dyslexic subjects had

refractive errors while seventy seven percent (77) were emmetropic (defined as

93

retinosocpic findings of less than 05 myopia less than +075 hyperopia or less than

075 astigmatism) (Fig 42) The distribution is as follows 65 (2 subjects) had

myopia 65 (2 subjects) had hyperopia (both latent) As cycloplegia was not

used in assessing refractive error an estimate of the diagnosis of latent hyperopia

was made based on a high difference between the retinoscopic findings and the

subjective refraction results In the event that the increased retinoscopic finding

(that is more plus) did not blur the subjectsrsquo vision latent hyperopia was assumed

About 10 (3 subjects) had hyperopic astigmatism 65 (2 subjects) had

amblyopia (defined as visual acuity in either eye of 2040 or a two-line difference

in acuity between the two eyes with no improvement with pinhole) 141 and 65 (2

subjects) had anisometropia (defined as a difference of ge 075 in sphere or cylinder

between the two eyes) (Fig 43)

ii Control Group Twenty two and a half percent (225) of the participants from

the control group had refractive errors classified as follows 65 (2 subjects)

had myopia 3 (1 subject) had hyperopia and 13 (4 subjects) had astigmatism

and 65 (2 subjects) had anisometropia No participant had amblyopia

Fig 42 Prevalence of Refractive Errors (total

01020304050607080

Myopia HyperopiaAstimatism (Hyperopic)Anisometropia Amblyopia Emmetropia

Prevalence

Dyslexic GroupControl Group

0

5

10

15

20

25

Prevalence

Refractive Errors (total)

Dyslexic GroupControl Group

94

Fig 43 Prevalence of the Types of Refractive Errors

Refraction data for two children from the dyslexic group who had cataracts could not be

obtained due to poor reflexes and were excluded from the analysis

The mean spherical equivalent (SE) refraction for the right eye for the dyslexic group was 086

098 and 070 103 for the control group was There was no statistically significant

differences between the two groups (p=066) The mean spherical equivalent refraction of the

left eye for the dyslexic group was 057 101 and 049 109 for the control group There

was no statistically significant difference between the two groups (p= 092) (Table 42)

Table 42 Descriptive Statistics for Refractive Error Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

RE (SE)

LE (SE)

DyslexicControl

DyslexicControl

2931

2931

086070

057049

098103

101109

-125-350

-150-350

500350

400350

066

092

423 Near Point of Convergence

i Dyslexic Group Thirty three percent (33 10 subjects) had NPC break points

(objective) of greater than or equal to 10cm while about sixty seven percent 67

(20 subjects) had NPCs of less than 10cm (Fig 44) One child complained of being

tired and did not participate in this procedure and was therefore excluded

ii Control Group Forty eight percent (48 15 subjects) had NPC break point

(objective) of greater than or equal to 10cm Fifty two percent (16 subjects) had

95

NPC less than 10cm (Fig 44) Data on NPC for one child from the dyslexic group

was lost

The mean NPC break was 890 503 for the dyslexic group and 1260 870 for the control

There was a statistically significant difference between the two groups (p = 0049) The mean

NPC recovery was 1400 588 for the dyslexics and 2200 820 for the control group

There was a statistically significant difference between the two groups (p = 006) (Table 43)

Table 43 Descriptive Statistics for NPC Break and Recovery Points Variable Group N (95 CI) Minimum Maximum P

Mean SD

NPC Break

NPC Recovery

DyslexicControl

DyslexicControl

3030

3030

8901260

14002200

503870

588820

500400

600800

26003400

28003800

049

006

424 Ocular Health

Six and a half percent (2 subjects) in the dyslexic group had cataracts (unclassified) on both

eyes No pathology was detected in the control group

425 Heterophoria

i Dyslexic Group No subject in the dyslexic group manifested with a phoria at

96

distance

At near 3 (1 subject) had an esophoria greater than or equal to 4 prism diopters

3 (1 subject) had an esophoria of 3 prism diopters 65 (2 subjects) had

exophorias of greater than or equal to 4 prism diopters 3 (1 subject) had an

exophoria greater than or equal to 8 prism dioptres and 65 (2 subjects) had

exophorias of 6 prism diopters (Fig 45)

ii Control Group At both distance and near no subject in the control group had

phoria of greater than 2 prism diopters

Fig 45 Prevalence of Phoria at Near (40cm) with Maddox Wing

Four children from each group could not complete the test as they left to attend lessons The

mean exophoria for the dyslexic group was 163 261 and 180 042 for the control group

There was no statistically significant difference between the two groups (p = 059) The

mean esophoria for the dyslexic group was 350 070 and 2 000 for the control group

There was no statistically significant difference between the two groups (p= 046) (Table 44)

No tropias were observed

Table 44 Descriptive Statistics for HeterophoriaVariable Group N ( 95 CI) Min Maxi P

97

Mean SD

At NearExophoria

Esophoria

DyslexicControl

DyslexicControl

2727

2727

163180

350200

261042

070000

000100

300200

1000200

400200

059

0469

426 Accommodation Functions

a Accommodation Posture As shown in Figure 46 the distribution of accommodation

lag is as follows

i Dyslexic Group

Thirty nine percent (11 subjects) had a lag of accommodation (defined as + 075 or

greater) three and a half percent (1 subject) had lead of accommodation (defined as

- 025 or greater) while fifty seven percent had normal accommodation posture

ii Control Group

Forty two percent (13 subjects) had lag of accommodation About eleven percent

(11) (3 subjects) had lead of accommodation while about thirty nine percent had

normal accommodation posture

Three children from the dyslexic group could not continue with this test as they had to

leave to attend class activities while three children from the control group sought

permission to be out of the testing room at that point These children were excluded

from the analysis for binocular accommodation lag

98

Fig 46 Prevalence of Accommodation Lag ( ge+075)

The mean accommodation lag for the right eye was 091 038 for the dyslexic group

and 092 057 for the right eye for the control group There was no statistically

significant differences between the two groups (p =083) The mean for the left eye for

the dyslexic group was 085 036 and 091 048 for the control group There was no

statistically significant difference between the two groups (p = 061) (Table 45)

Table 45 Descriptive Statistics for Accommodative PostureVariable Group N (95 CI) Minimum Maximum P

Mean SD

Accomm lag

RE

LE

DyslexicControl

Dyslexic Control

2828

2828

091092

085091

038057

036048

000-050

-050-050

200200

125200

083

061

b Accommodative Facility As shown in Figure 47 the distribution of accommodation

facility was as follows

i Dyslexic Group

Forty six percent (46 13 subjects) had normal accommodative facility (defined as

0

10

20

30

40

50

Prevalence

Bi no cu lar A ccom m od ation Lag

D y s lex ic G r o u p

C on t ro l G ro u p

99

greater than 7cpm) binocularly at near while (54) (15 subjects) had

accommodation facility less than or equal to 7cpm (Fig 47)

ii Control Group

Sixty seven percent (67) (18 subjects) had normal accommodative facility

(defined as greater than 7cpm) while thirty three percent (33) (9 subjects) had

accommodation facility less than 7cpm

Three children from the dyslexic group and four from the control did not complete the test

as they indicated that they were tired The mean binocular accommodation facility was

686 cpm 274 for the dyslexic group and 885 369 for the control group There was a

statistically significant difference between the two groups (p=0027) (Table 46)

Table 46 Descriptive Statistics for Binocular Accommodative FacilityVariable Group N ( 95 CI ) Minimum Maximum p

Mean SDAccommodativeFacility at near

DyslexicBE

ControlBE

28

27

686

885

274

369

2

2

12

21

002

7

c Amplitude of Accommodation Monocular amplitude of accommodation in both eyes

and for both groups did not differ significantly (not more than 050D) except for one

subject who had a difference of 4 diopters between the two eyes The amplitude of

100

accommodation for two subjects who had latent hyperopia in the dyslexic group was

relatively low (600DS and 800DS) as compared to age minimum amplitude of

accommodation of 1175DS The amplitude of accommodation for two participants who

had cataracts were excluded from the analysis

Only the data for the monocular amplitude was analyzed When accommodation

amplitude is assessed monocularly it measures the response for each eye individually

and is particularly important to determine whether a patient has accommodative

insufficiency 133 The monocular amplitude of accommodation has been used to assess

amplitude of accommodation function in several studies 86 88100102-103 The mean for the

right eye for the dyslexic group was 1198 234 and 1287 108 for the control group

There was no statistically significant difference between the two groups (p =007) (Table

47) The mean for the left eye for the dyslexic group was 1214 214 and 1287 116

for the control group There was no statistically significant difference between the two

groups (p =022) (Table 47)

Table 47 Descriptive Statistics for Amplitude of Accommodation Variable Eye Group N ( 95 CI ) Minimum Maximum p

Mean SDAmplit of Acomm

RE

LE

DyslexicControl

DyslexicControl

2931

2931

11981287

12141287

234108

215116

810

810

2015

2015

007

022

d Relative Accommodation (NRA PRA) As shown in Fig 48 the results for the relative

accommodation were as follows

For the PRA all subjects in both groups had PRAs of more than -200DS For the

negative relative accommodation (NRA) 89 of the dyslexic subjects had NRA greater

than +200DS while 96 of subjects from the control group had NRA greater than

+200DS

101

Fig 48 Prevalence of Relative Accommodation (NRA+2PRA-200)

Three children from the dyslexic group and five from the control group could not

continue with the test for the assessment of relative accommodation as they had to attend

important class activities

The mean PRA for the dyslexic group was -623 117 and -606 063 for the control

group There was no statistically significant difference between the two groups (p

=051) The mean NRA for the dyslexic group was 322 079 and 311 047 for the

control group There was no statistically significant difference between the two groups

(p =068)(Table 48) The findings for RA were higher than the published norm This

may be due to the process of addition of trial lenses

Table 48 Descriptive Statistics for Relative Accommodation Variable Group N ( 95 CI ) Minimum Maximum p

Mean SDPRA

N RA

DyslexicControl

DyslexicControl

2826

2826

-623-606

322311

117063

079047

-900-700

200200

-400-500

60045

051

068

427 Fusional Reserves

The children in both groups either could not report or understand blur so the result for break

102

and recovery was used in all analysis of vergence function

a Base-in Vergence Reserves at Distance two children from the dyslexic group could not

complete all aspects of the fusional reserves assessment because they were tired These

children were excluded in the analysis

The mean base-in to break for the dyslexic group was 1469 683 and 1600 350 for

the control group There was no statistically significant difference between the two

groups (p =046) The mean base in to recovery for the dyslexics was 1172 620 and

1280 317 for the control group There was no statistically significant difference

between the two groups (p =049) (Table 49)

Table 49 Descriptive Statistics for Base-in Vergence Reserves at distanceVariable Group N (95 CI) Minimum Maximum P

Mean SD

BI to Break (dist)

BI to Recovery (distance)

DyslexicControl

DyslexicsControl

2931

2931

14691600

11721280

683350

620317

410

28

4022

3520

046

049

b Base-in Vergence Reserve at Near The mean base in to break at near was 1185 514

for the dyslexics and 1283 313 for the control group There was no statistically

significant difference between the two groups (p=029) The mean base in to recovery was

872 478 for the dyslexics and 1032 335 for the control group There were no

statistically significant differences between the two groups (p=017) (Table 410)

Table 410 Descriptive Statistics for Base-in Vergence Reserves for NearVariable Group N (95 CI) Minimum Maximum P

Mean SDBI to Break (near)

BI to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

11851283

8721032

514313

478335

26

14

2518

2015

029

017

103

c Base-out (BO) Vergence Reserves at Distance Four children from the control

group could not give a report on the recovery point Two children from the dyslexic group

could not complete the test The mean base out to break at distance for the dyslexic group

was 2706 925 and 2416 975 for the control group There was no statistically

significant difference between the two groups (p=024) The mean BO to recovery for the

dyslexic group was 1876 796 and 17 693 for the control group There was no

statistically significant difference between the two groups (p= 040) (Table 411)

Table 411 Descriptive Statistics for Base-out Vergence Reserves at Distance Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

BO to Break(Distance)

BO to Recovery(dist)

DyslexicControl

DyslexicsControl

2931

2927

27062416

18761700

925975

796693

10001000

400600

0004000

35003500

024

040

d Base-out Vergence Reserves at Near The mean base-out to break at near was 2160

1162 for the dyslexic group and 2109 842 for the control group There was no

statistically significant difference between the two groups (p =084) The mean base-out

to recovery at near for the dyslexic group was 1335 745 and 1555 625 for the

control group There was no statistically significant difference between the two groups

(p =016) (Table 412) The comparative and descriptive statistsics for all variables is

shown in Table 413

Table 412 Descriptive statistics for base-out vergences at near Variable Group N (95 CI) Minimum Maximum P

Mean SD

BO to Break (near)

BO to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

21602109

13351555

1162842

745625

8001000

600800

4040

3530

084

016

104

Table 413 Comparative Descriptive Statistics for Dyslexic and Control groups

Variable Dyslexics( 95 CI)

Control( 95 CI)

N Mean SD N Mean SD PVisual Acuity

RE 31 017 031 31 000 024 029LE 31 020 033 31 000 024 023

Refractive ErrorRE 29 086 098 31 070 103 066LE 29 057 101 31 049 109 009

NPC Break 30 890 503 31 1260 870 049NPC Recovery 30 14 588 31 2200 820 006

Heterophoria (Near)Exo 27 163 261 27 180 042 059Eso 27 350 070 27 200 000 046

PRA 28 -623 117 26 -606 063 051NRA 28 322 079 26 311 047 068

Amp of AccommodationRE 29 1198 234 31 1287 108 007LE 29 1214 215 31 1287 116 022

Accom Facility (bin) 28 686 274 27 885 369 003

Accomm LagRE 28 091 038 31 092 057 083LE 28 085 036 31 091 048 061

BI Break (Dist) 29 1469 683 31 1600 350 046BI Recovery (Dist) 29 1172 620 31 1280 317 049

BI Break (Near) 29 1185 514 31 1283 313 029BI Recovery (Near) 29 872 478 31 1032 335 017

BO Break (Distance) 29 2706 925 31 2416 975 024BO Recovery (Distance) 29 1876 796 31 1700 693 040

BO Break (Near) 29 2160 1162 31 2109 842 084BO Recovery (Near) 29 1335 7 45 31 1555 625 016

105

Summary Statistical tests were performed on the data to ensure scientific validity and were presented for

comparative and descriptive purposes The prevalence of visual acuity was similar between the

dyslexic and control groups and there was no statistically significant difference between the two

groups (p = 029 right eye 023 left eye)

The prevalence of total refractive errors was similar between the two groups and there was no

statistically significant difference between the two groups (p = 066 right eye and 092 left

eye) Hyperopia was slightly more prevalent in the dyslexic group than in the control group

The prevalence of myopia was the same in both groups Astigmatism was more prevalent in

the control group than in the dyslexic group The prevalence of anisometropia was the same in

both groups Amblyopia was more prevalent in the dyslexic group compared to the control

group

The control group had a higher near point of convergence break and recovery points than the

dyslexic group The NPC showed a statistically significant difference between the two

groups NPC break (p =0049) and NPC recovery (p =0006) The prevalence of exophoria at

near was higher in the dyslexic than the control group There was no statistically significant

difference between the two groups (p= 059) The prevalence of esophoria in the dyslexic

group was higher than that found in the control group but there was no statistically significant

difference (p =046)

The prevalence of relative accommodation was similar in both groups and there was no

statistically significant difference between the groups The dyslexic group had a reduced

amplitude of accommodation compared to the control group and there was no statistically

significant difference between the two groups (p=007 right eye andp = 022 left eye) The

dyslexic subjects performed significantly worse than the control subjects in accommodative

facility function and there was a significant difference between the two groups (p =0027)

The dyslexic subjects had a lower accommodation lag than the subjects from the control

group There were no statistically significant differences between the two groups right eye (p

= 083) and left eye (p = 060)

At near the negative fusional vergence (base in vergences)) was reduced in the dyslexics

compared to the control group while the positive fusional vergence (base out vergences) was

similar for both groups There was no statistically significant difference between the groups

106

(p = 021 for BI break at near) and (p = 084 for BO break at near) Having analyzed the

results Chapter Five will discuss the findings in the context of the international studies

reviewed in the literature

107

CHAPTER FIVE DISCUSSION

51 Introduction

A review of the literature reveals that studies 2356878990 conducted on dyslexic children have

investigated the prevalence of vision defects in the dyslexic population and examined whether

such vision anomalies are correlated with dyslexia by comparing vision characteristics in

dyslexic subjects to normal readers

The aim of this study was to determine the prevalence of vision defects in a South African

population of the dyslexic school children and investigate the relationship between vision and

dyslexia by comparing the vision characteristics of the dyslexic to the control group of learners

from a mainstream school The proposed null hypothesis was that there was no statistically

significant difference between the means of the vision functions in the dyslexic population

compared to the control group A discussion of the results of this study is presented

52 Prevalence of Vision Defects

521 Visual Acuity

The 32 prevalence of visual acuity worse than 69 was the same for both groups A reduced

visual acuity could be due to ocular abnormality (such as cataract) or due to normal refractive

error differences in the population Visual acuity defects due to ocular diseases are usually not

common in paediatric populations142 but tend to be related to refractive error changes in the

population143 This relates more in the present study as the prevalence of defects of visual acuity

and the total refractive errors were similar Consequently in relating visual acuity to refractive

errors myopia astigmatism and hyperopia may reduce distance or near visual acuity

108

Visual acuity testing is widely performed in screening refraction and monitoring of disease

progression It is also used in both basic and clinical vision research as a way to characterize

participants visual resolving capacities144 Specific to the present study visual acuity testing

helps detect visual problems at the distances at which most school learning activities occur and

gives useful preliminary information on how good or poor a childrsquos vision is reduced distance or

near visual acuity gives an indication of poor vision that will necessitate further examination

However in assessing how visual acuity relates to reading performance it is important to note

that visual acuity as measured with the traditional letter identification only assesses the ability to

discern letters on the visual acuity charts and does not assess dynamic vision behaviour

binocular eye control or ocular stress and so may not necessarily mean normal vision

Furthermore normal visual acuity may not necessarily mean normal vision since some people

may have other vision defects such as color vision or reduced contrast or inability to track fast-

moving objects and still have normal visual acuity The reason visual acuity is very widely used

is that it is a test that corresponds very well with the normal daily activities a person can handle

and evaluates their impairment to do them 145

In relation to previous studies the visual acuity findings in this study are similar to reports by

Buzzelli 6 Goulandris et al 90 and Metsing and Ferreira 103 On the contrary Hoffman 83 found

a lower prevalence (1028) of visual acuity worse than 69 in a group of learning disabled

children In contrast to the present study however the mean age of subjects in Hoffmanrsquos study

was 8 years 6 months while the mean age in the present study was 13 for dyslexics and 11years 9

months for the control group Grnlund et al 140 documented that age must be taken into

consideration when describing and comparing VA in different populationsbecause VA develops

from birth to adolescence A lower prevalence (10) of visual acuity worse than 69 was also

reported by Sherman84 Latvala et al 3 reported a slightly worse distance acuity in the dyslexic

group than in the control group but noted no statistically significant difference Evans et al 5

found that dyslexic groups had a significantly worse visual acuity than the control group

(Binocular distance VA p =00164 and binocular near VA p =00018) Similarly Ygge et al2

reported that the subjects from the control group had a better visual acuity than the dyslexic

group at both distance and near and that the results showed statistically significant differences

(distance at p =003) (near at p=0005) Bucci et al 89 and Kapoula et al 87 reported the

109

prevalence of visual acuity of all participants in their study to be normal A study conducted by

Grisham et al 86 lacked a control group but it was reported that 568 of the participants had 66

acuity in both eyes (compared to 68 in dyslexic group in the present study) 172 had 69 or

worse (compared to a total of 315 who had visual acuity of 69 and worse in the present study)

The mean age of the participants in the study by Grisham et al 86 was 154 years while the mean

age of the participants in the dyslexic group in the present study was 13 years

The factors which may affect the outcome of visual acuity measurements that may lead to

variations in results reported by different authors include size position and distribution of retinal

mosaics (retinal photoreceptors) 143 optical aberration pupil size clarity of the optical media

age method and type of chart used contrast isolated or multiple letters state of accommodation

illumination as well as the criteria used to define visual acuity114 140 Psychological factors

affecting visual acuity testing results include blur interpretation fatigue and malingering 143

These factors may be difficult to control and could constitute confounding variables which may

lead to variations in visual acuity results reported by different authors

522 Refractive Error

The prevalence of uncorrected refractive error of 23 found in the dyslexic group was similar to

the 225 found in the control group and there was no statistically significant difference between

both groups (p = 066 right eye p = 092 left eye) This finding may mean that the dyslexic

subjects are not at more risk of a particular refractive anomaly compared to participants from the

control group as the dyslelxic gourp has similar distribution of refractive errors

Some studies 5838498103146 did not classify refractive error according to the types but presented

the results for the total refractive error On this basis the prevalence of total refractive error

(23 for dyslexic and 225 for the control group) in the present study is similar to reports by

other studies Hoffman83 reported a 215 prevalence of refractive error in a study of the

incidence of vision difficulties in learning disabled children Although there was a marked

difference in the sample size in both studies the similarities in the findings may be because as

with the present study Hoffmans study was based on a population of learners in a special school

110

referred for optometric care by educators psychologists and reading specialists

Similar to the present study Evans et al 5 and Ygge et al 2 reported that the prevalence of

refractive errors in the dyslexic and control groups was similar Metsing and Ferreira103 found

no statistically significant difference between the learning disabled and the mainstream

groups in their study Contrasting findings were reported by Sucher and Stewart 98 who found

the prevalence of refractive errors to be 94 for the control group although the 208 found in

their dyslexic group is similar to the 23 found in the present study Wesson 146 reported the

prevalence of refractive error in children with reading dysfunction to be 38 while the

prevalence of refractive errors was 16 in the study by Sherman 84 In the study by Alvarez and

Puell 88 the prevalence of refractive error was 424 in the dyslexic group while 415 of the

control group had refractive error When compared to the present study the higher prevalence of

refractive error in the dyslexic group can be accounted for by the relatively high prevalence of

hyperopia (287) and in the control group the higher prevalence of refractive error can be

accounted for by the high prevalence of myopia (194) It is unclear why the prevalence of

hyperopia was as high as no information was given on how refractive error was assessed

Statistically for the present study the mean refractive errors for the dyslexic group were right

eye 086 098 left eye 057 1013 and right eye 070 103 left eye 049 109 for the

control group There was no statistically significant difference (RE p =066 LE p = 092)

Evans et al 91 reported a mean spherical refractive error of 077DS (right and left eye similar) In

the study by Alvarez and Puell 88 the refractive error results (descriptive statistics) reported

were as follows for the poor readers mean spherical equivalent refractive errors for the right and

left eye were 020 06 and 020 06 respectively In the control group mean spherical

equivalent refractive errors for the right and left eye were - 020 08 and - 014 08

respectively The descriptive statistics on refractive errors was not reported in the studies 3 93

cited earlier which made it impossible to compare the statistical findings with the present study

In the present study about 68 of participants from both groups were emmetropic (defined as lt -

050DS +100DS and 075D cyl) while 655 of the dyslexic children and 645 of children

from the control group were emmetropic in the study by Alvarez and Puell 88 Only the study by

111

Alvarez and Puell 88 reported on emmetropia

Refractive errors are among the leading causes of visual impairment worldwide and are

responsible for high rates of visual impairment and blindness in certain areas School children

are considered a high risk group because uncorrected refractive errors can affect their learning

abilities as well as their physical and mental development147

Several factors may lead to variations in results found in different studies This include the type

of population studied (clinical or non-clinical) sampling method (convenience or cluster)

classification criteria examiner bias and more specifically the use of cycloplegia in the

assessment of refractive errors

The following studies2 3 8895 categorized refractive errors as hyperopia myopia astigmatism

anisometropia or amblyopia

a Hyperopia In the present study hyperopia appeared slightly more prevalent (65) among

the dyslexic group than in the control group (3) This result agrees with the report from

other studies 93 95 and Alvarez and Puell88 who reported a much higher prevalence (287)

for the dyslexic group compared to 161 of the control group Helveston85 found no

difference in the prevalence of hyperopia between the population of normal and poor

readers

Hyperopia is the refractive error that is consistently reported 3939598 to be associated with

reading difficulties so it was expected that one will find the higher prevalence of hyperopia

in the present study

In the present study two children had latent hyperopia (based on the assumption that

increased plus did not blur the distance vision) The full magnitude of the hyperopic

findings could not be estimated because cycloplegia was not used Therefore it is possible

that the prevalence of hyperopia may have been under-estimated Williams et al 148 studied

the ldquoprevalence and risk factors for common vision problems in childrenrdquo and concluded

112

that the 48 prevalence of hyperopia in their study was ldquoan underestimate as data was

collected without cycloplegiardquo Borghi and Rouse (cited by Scharre and Cree)149

determined that on an average 063D more plus was identified by cycloplegic refraction

In relating hyperopia to reading difficulties simple to moderate hyperopia may not cause

constant blur at a distance or near point but the extra accommodative effort produces

asthenopic symptoms of intermittent blur headache fatigue and loss of concentration and

inattention in some patients which may be mistaken for short attention span Uncorrected

hyperopia is associated with esophoria at near point which can stress the fusional vergence

systems that hold the eyes in correct alignment If the hyperopia and esophoria is excessive

an accommodative esotropia can result 81150 This may result in difficulty in reading

b Myopia The 65 prevalence of myopia was the same in both groups This result is similar

to an equal prevalance of myopia reported by Eames 95 (4 in both groups) Alvarez and

Puell 88 reported that 57 of dyslexics were myopic while 194 of the control group was

myopic The prevalence of myopia was higher in studies by Rosner and Rosner 93 (54

non- learning disabled and 19 learning disabled) and lower than the prevalence reported

by Grisham and Simons150 In contrast to the present study subjects from the study by

Rosner and Rosner 93 were patients of an eye clinic where the two main reasons for referral

were reduced visual acuity and school learning problems while subjects for the present

study were referred to the special school mainly due to low academic performance which

may not necessarily be vision- related The subjects were chosen for the study irrespective

of whether they complained of a visual impairment or not whereas in the study by Rosner

and Rosner 93 study subjects had been identified as having a visual impairment Therefore

the higher prevalence may have been from the selection protocol used by Rosner and

Rosner 93 in their study

The prevalence of myopia was not reported in several studies 3568498146 referenced but was

presented as ldquorefractive errorsrdquo (was not classified as myopia hyperopia or astigmatism)

The onset and progression of myopia may be influenced by factors such as environment

nutrition genetic predisposition premature and low birth weight the effect of close work

113

racial and cultural factors differences in pupil size and illumination143151 Variations in the

prevalence of myopia reported by different authors may be related to the variation in the

above-mentioned factors

c Astigmatism

The prevalence (13) of astigmatism in the control group was higher than in the dyslexic

group (10) and there was no statistically significant difference between the two groups

Similarly Alvarez and Puell 88 found a higher prevalence of astigmatism in the control

group compared to the dyslexic group astigmatism was detected in 161 from the control

group compared to 8 and 92 right and left eyes respectively from the dyslexic group

In contrast Latvala et al 3 reported that 36 in the dyslexic group had astigmatism and

none from the control group had astigmatism Ygge et al 2 reported that the prevalence of

astigmatism was higher in the dyslexic group (28 left eye 25 ) compared to the control

groups (right eye 183 left eye 243) although there was no statistically significant

difference between the groups (p=025)

Rosner and Rosner93 reported that 30 of non-learning disabled and 27 of learning

disabled participants had astigmatism which differed markedly from the findings in the

present study despite the similarities in the study population Eames95 found an equal

prevalence of astigmatism (7) between the two groups studied

Astigmatism affects vision in different ways Clinically astigmatism over 150 diopters can

often cause severe eye strain and interfere with reading and even lesser degrees of

astigmatism can be symptomatic in some patients 81

Naidoo et al 157 studied refractive error and visual impairment in African school children in

South Africa Although the study was conducted specifically on mainstream school

children it relates to the present study as it provided useful information on the visual

characteristics of school children in South Africa and forms a basis for comparison since all

the studies reviewed in the present study were conducted on Caucasian population except

114

for the study by Metsing and Ferreira 103 which did not provide much information on

refractive errors in the their study of learning disabled children in Johannesburg South

Africa

Naidoo et al 157 reported a prevalence of hyperopia of 18 myopia of 29 astigmatism

of 67 and 68 right eye and left eye respectively In comparison the prevalence of

refractive error for the control group in the present study was hyperopia 3 myopia 65

and astigmatism 13 The refractive error trend (astigmatism being higher in prevalence

than myopia and hyperopia) was similar between the present study and the study by Naidoo

et al 157 despite the difference in prevalence reported Given that ethnic origins culture

socio-economic class are comparable between both studies the difference in prevalence

may (partly) have been due to different sampling methods (cluster versus convenience) and

the use of cycloplegia in their study The use of cycloplegia has been reported to yield

spherical aberrations and unpredictable errors due to associated mydriasis 110

It has also been documented 158 that the use of cycloplegia yields more positive retinoscopic

findings although this did not seem to apply in this comparison as the prevalence of

hyperopia in the present study was higher than that reported by Naidoo et al 157 A possible

explanation for the difference in prevalence may be related to the different criteria used to

define hyperopia Bennet et al 159 stated that the degree of ametropia ldquohellip considered to be

within normal limits in a particular study will drastically affect the prevalencerdquo Similar

opinion was noted by Gronlund et al 140 who stated that ldquothe choice of definition will of

course have a great impact on prevalence In their study on Swedish children aged 4-15

years Gronlund et al 140 defined astigmatism as ge 075 and stated that the prevalence of

astigmatism would have been 21 (instead of 32) had they defined astigmatism as ge

100 Furthermore Grisham et al 86 in their ldquoStudy of poor readers in high schoolrdquo noted

that helliprdquoif one selected less stringent criteria then the proportion of students with any given

dysfunction would decreaserdquo In the present study the criteria used to classify hyperopia

were ge +100D while Naidoo et al 157 used ge +200DS cut- off

115

d Anisometropia Anisometropia may be defined as a difference of 075 (sphere or cyl) or

more between the two eyes and this definition was the criteria used in this study 129

Anisometropia is of great clinical interest because of its intimate association with

strabismus and amblyopia141 152

The 65 prevalence of anisometropia was similar in both groups This is comparable to

results reported by Latvala et al 3 where the prevalence of anisometropia was 36 (2

subjects) for the dyslexic group and 6 (3 subjects) for the control group but contrasts with

findings by Eames 95 who reported the prevalence of anisometropia to be 13 for poor

readers and 6 for the control group while Ygge et al 2 found a higher prevalence of

anisometropia in the control group (158) than in the dyslexic group (94)

In anisometropia the difference in refraction as well as the refractive error causes the image

to be out of focus on one retina blunting the development of the visual pathway in the

affected eye 141 The fovea of an anisometropic eye receives images from the same visual

object however the images from the more myopic or hyperopic eye are out of focus 153 It

appears that anisometropia is a major cause of amblyopia for at least one third of all

amblyopias 131 and anisometropia is considered to be a causal factor in the pathogenesis of

amblyopia and strabismus It is also estimated that 6 to 38 of all cases of amblyopia are

caused by anisometropia without strabismus whereas in about 12 to 18 of the children

with strabismus this is accompanied by anisometropia 154

According to Grosvenor 114 uncorrected anisometropia even of a moderate amount may

induce eyestrain because it is impossible for the accommodation mechanism to maintain

clear images on the retina at the same time On the other hand large amounts of

anisometropia (2DS or more) may not cause symptoms as no effort is made to maintain

single binocular vision 114

Rutstein and Corliss155 investigated the relationship between anisometropia amblyopia and

binocularity and concluded that higher degrees of anisometropia generally cause deeper

116

amblyopia and poorer levels of binocularity for hyperopes but not for myopes and stated

that ldquoAlthough uncommon small amounts of anisometropia can cause moderate

amblyopiardquo However the results of the present study failed to support this claims as none

of the subjects had amblyopia resulting from anisometropia

In anisometropia the displacement or distortion of the image prevents the development of

fine visual perception in the occipital cortex and puts the child at risk for developing

amblyopia 141 Anisometropia causes poor reading skills probably through the mechanism

of poor sensory and motor fusion rather than reduced visual acuity It degrades binocular

coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 150

e Amblyopia Two subjects (65) in the dyslexic group had ambyopia due to cataracts and

none from the control group had amblyopia This result may be comparable to studies by

Latvala et al 3 who reported 36 (2 subjects) for the dyslexic group and that of Rosner and

Rosner 93 with a prevalence of 4 for the non-learning disabled group and 3 for the

learning disabled group Cataract is a major cause of amblyopia (deprivation ambyopia) by

causing an impediment to the visual axis 141 Other conditions that may lead to amblyopia

include strabismus (which causes the images on the retinas to be dissimilar) and uncorrected

refractive errors (especially as in anisometropia) Anisometropia is related to amblyopia

Amblyopia is a major public health problem It is the most common cause of monocular

vision loss in children and young adults Early recognition and prompt referral are crucial

especially during infancy and childhood to prevent permanent loss of

vision 141

523 Heterophoria

Heterophoria is the relative deviations of the visual axes when the eyes are dissociated 160 Some

degrees of heterophoria are considered normal for persons with normal binocular vision

Approximately 1-2 prism dioptres of esophoria or 1- 4 diopters of exophoria at distance should

117

be considered to be within normal limits161 At near 3-6 prism dioptres of exophoria is

considered normal (physiological exophoria) 161 A normal healthy eye is usually able to

overcome these small deviations and so it is described as being compensated161 However

esophoria is much less compatible with comfortable vision than is exophoria Any amount of

esophoria in a patient with visually related symptoms may be problematic 162

In the present study all subjects were orthophoric at distance This report corroborates reports by

other authors Mathebula et al 163 studied heterophoria in a South African population of school

children (mean age 10 16 years) and reported that the mean distance horizontal heterophoria

showed orthophoria Similarly Evans et al 100 reported that all subjects examined ldquowere

orthophoric at distance with cover testrdquo According to Dowley160 ldquothe high prevalence of

orthophoria is realrdquo and that ldquothe high prevalence of distance orthophoriahellip may be due to the

coordinating influences of the same mechanisms that subserves prism adaptationrdquo A similar

view was expressed by Walline et al 164 who studied the ldquodevelopment of phoria in childrenrdquo and

stated that ldquonearly all of our subjects were orthophoric at distancerdquo

The major findings in heterophoria was a 95 prevalence of exophoria at near which was more

in the dyslexic subjects than in the control group with no exophoria There was no statistically

significant difference between the two groups (p= 059) At near 3 (1 subject) of the dyslexic

group had an esophoria greater than or equal to four prism diopters An interpretation of the

above finding is that the higher prevalance of heterophoria may mean that the dyslexic subjects

may be more uncomfortable when doing near work than the nroaml readers A possible

explanation may be that according to von Noorden (cited by Kommerell et al)165 heterophoria

typically causes asthenopia Patients with asthenopic symptoms (feeling of heaviness dryness

and soreness of the eyes pain in and around the eye frontal and occipital headache and the eyes

are easily fatigued) often have an aversion to reading and studying Typically such complaints

tend to be less severe or to disappear when patients do not use their eyes in close work165 It has

been documented that decompensated heterphoria (heterophoria accompanied by symptoms) can

be due to poor fusional reserves problems with sensory fusion (such as in anisometropia) or may

be due to an unusually large phoria which may be due to anatomical reasons or uncorrected

hyperopia 161166 The presence of phoria could contribute to visual and attention abnormalities

118

noted among children with reading difficulties making near point visual activities more

strenuous

Nearpoint esophoria has been reported to be associated with childhood myopia progression 164167

In the present study the only subject that had four prism diopters esophoria at near in the

dyslexic group had +125 hyperopia while the subject that had 3 prism diopters esophoria had -

150 DS myopia However due to the small sample size in the present study it is difficult to

draw any conclusion on the association between myopia and esophoria

The findings on heterophoria as in other aspects of vision functions in the dyslexic populations

reviewed are mixed However similar to the present study Latvala et al 3 assessed near

heterophoria using the Maddox Wing and reported a higher prevalence of near exophoria in the

dyslexics than in the control group Similarly Kiely et al 12 documented that dyslexics tend to

be more exophoric than normal readers Evans et al 101 found a higher prevalence of lateral

phoria in learning disabled subjects compared to normal readers In contrast to the above

findings Sucher and Stewart 98 assessed phoria using the cover test (testing distance was not

indicated in the results) but found a slightly higher prevalence of phoria in the control group than

in the dyslexics Evans et al 100 assessed near horizontal phoria using the Maddox Wing as in

present study but found no difference in near horizontal phoria between the dyslexic and control

groups Bucci et al 89 reported that there was no difference in phoria results at both distance and

near between the dyslexic and control groups Metsing and Ferreira 103 found no statistically

significant difference between the learning disabled and the mainstream groups on

heterophoriaThe use of the cover tests for both distance and near to assess heterophoria may have allowed for

standardization in technique but may not have implied that more valid results will be obtained

The cover test and the Maddox Wing have been used simultaneously to assess heterophoria in

other studies3 100 The possible sources of variations in results reported by different investigators

include classification criteria and poor technique 168

524 Near Point of Convergence

119

In the present study the prevalence of a remote (greater than 10cm) near point of convergence

was higher in the control group with 48 than in the dyslexic group with a prevalence of 33

There was a statistically significant difference in the near point of convergence break (p=0049)

and recovery (p = 0006) between the two groups

It may be that children who do not have reading difficulties tend to read more often than children

who experience difficulty to read This is because with increasing ability to read there is likely

to be more demand on accommodation and convergence resulting in near point stress as well as

esophoria The normal readers use and exercise the accommodation and convergence system

more frequently According to Owens and Wolf-Kelly 169 near work has long been considered a

potential source of visual problems stating that in their study reading ordinary text at a near

distance for about one hour induced significant changes in the resting postures of both

accommodation and vergences The authors indicated that ldquonear work induces a recession of the

near point of accommodation or vergencesrdquo

The near point of convergence break is associated with changes in inter-pupillary distance with

age As inter-pupillary distance widens with physical growth the amount of convergence in

prism diopters increases for a given distance from the subject 132 Chen et al 170 studied near point

of convergence in children aged 1-17 years and reported that an increasing incidence of remote

near point of convergence with increasing age in their study might be due to the near work

demands of primary school which might create a different level of near point stress than the near

work conditions in pre- primary school years

Similar to the findings in the present study Evans et al 91 reported a statistically significant

relationship between NPC and reading retardation (p=0019) Contrasting findings were reported

by other authors Bedwell et al 171 studied visual and ocular anomalies in relation to reading

difficulties and reported that 466 of the group of ldquogood readersrdquo had abnormal near point of

convergence compared to 58 of ldquopoor readersrdquo Similarly Bucci et al 89 and Kapoula et al 87

reported that the near point of convergence was significantly more remote in dyslexics than the

control In the study by Latvala et al 3 1275 of the dyslexic group and only 2 from the

120

control group had a near point of convergence of worse than 8cm A statistically significant

difference between the two groups was reported (p=00385) Metsing and Ferreira 103 found

no statistically significant difference between the learning disabled and the mainstream

groups on the near point of convergence function In a study that lacked control subjects

Grisham et al 86 reported a lower prevalence (154) of NPC (9cm or further) in a group of high

school poor readers The authors measured NPC three times on each subject in other to detect

fatigue but stated that it should therefore be noted that fatigue may be a problem for poor

readers even if they are able to converge at 8 cm if they tire easily then the ability to sustain

convergence during longer periods of reading may suffer

In the present study the measurement of the near point of convergence was repeated three times

on each subject in order to detect visual fatigue that the children experience everyday This

approach was documented by other authors 868789 111-113 Some authors111-113 have recommended

the measurement of the near point of convergence several times in order to detect fatigue which

is often indicative of poor convergence fusion system which may affect distract a child from

reading 172

525 Accommodation Functions

a Accommodation Facility Accommodative facility assesses the rate at which

accommodation can be stimulated and inhibited repeatedly during a specific time period It

relates to the individualrsquos ability to shift focus quickly and efficiently for varying distances

and is extensively used in the reading process 6

Only the result for the binocular accommodative facility was recorded and was used for analysis

in the present study Siderov and Johnston 174 noted that monocular accommodation

measurements provides a direct evaluation of the dynamics of accommodative response while

binocular testing of accommodative facility provides similar information but also reflects the

interactive nature of the relationship between accommodation and vergences

For the dyslexic group 54 (15 subjects) had inefficient accommodation facility while only 33

121

(9 subjects) of the control group had inefficient accommodative facility Statistically the control

group had a better accommodative facility (885cpm 369) than the dyslexics (686cpm 274)

and there was a statistically significant difference between the two groups (p =0027) It has

been documented 100 that an abnormal accommodative facility could imply difficulty in changing

focus from far to near and subsequently lead to a lack of interest in learning An efficient facility

of accommodation is particularly important because a greater period in school involves changing

focus between the chalkboard to near task such as writing at a desk Low values of

accommodative facility have been associated with symptoms related to near point asthenopia134

The result of the present study corresponds with findings by Evans et al 100 who reported that

dyslexics appeared to be slower at a test of accommodative facility Similar results were reported

in other studies 128898 but contrasts with results reported by Buzzelli 6 Buzzelli 6 assessed

accommodation facility using a Bernell Vectogram that utilized a polarised target to control for

suppression Buzzelli6 assessed accommodation facility as follows patients were asked to clear

the number 9 on the Bernell Vectogram target at 40cm They alternately viewed the target

through a 2D lens The test was continued for 20 cycles (a cycle being clearing the plus and

minus lens each presented once) The total time in seconds was recorded for the right eye left

eye and both eyes The polarized target controlled for suppression The use of the vectogram

to assess accommodation facility by Buzzelli6 differs from the flipper technique that was used in

the present study Secondly there was no control for suppression in the present study The

difference in results found between the two studies may be due to the lack of control for

suppression in the pre sent study Siderov and Johnston174 stated that ldquoclinical measurements of

binocular accommodation facility could vary depending on whether or not suppression has been

monitoredrdquo

A total of 317 of the poor readers in the study by Grisham et al 86 had inadequate (9cm or less)

accommodative facility

Allison173 suggested that there are different norms submitted by different authors for

accommodative facility measurements and that apart from adhering strictly to recommended

norms for accommodative facility testing other factors to be considered when evaluating

accommodative facility include the difficulty and speed of responses to the plus and minus lenses

122

and whether the subjects fatigue easily 173 Kedzia et al (cited in Zadnik)175 documented that

the factors that affect the testing of accommodative facility include the size and position of the

text being read the complexity of the target reaction time of the child to call out the symbol and

the magnification or minification factors introduced by the lenses themselves The above

variables are factors to be considered in assessing the variations in the findings for

accommodative facility reported in this study as compared to other studies

b Amplitude of Accommodation Accommodation is the ability to adjust the focus of the

eye by changing the shape of the crystalline lens to attain maximum sharpness of the retinal

image of an object of regard The absolute magnitude of the accommodative response is

termed the accommodative amplitude 120

Statistically the dyslexic group had a slightly reduced monocular amplitude of

accommodation (mean 12D) compared to the control group (1287D) but there was no

statistically significant difference between the two groups (p=070) The slight difference

in the mean amplitude of accommodation between the two groups may have been due to the

two subjects from the dyslexic group who had latent hyperopia

Similar to the present study lvarez and Puell88 reported that monocular accommodative

amplitude was significantly lower in the group of poor readers Evans et a l91100 in two

seperate studies 91100 reported that amplitude of accommodation was reduced in the

populations studied Ygge et al 102 and Goulandris et al 90 found no statistically significant

difference between the dyslexic and control groups Grisham et al 86 found that 247 of

the children had amplitude of accommodation which the authors classified as ldquoweakrdquo or

ldquovery weakrdquo while 115 had accommodative amplitudes classified as ldquoborderlinerdquo In the

present study the mean amplitude of accommodation for each group was within normal age

norms according to the amplitude norms and Hofsteterrsquos formula 106161

In the study by Metsing and Ferreira 103 a higher percentage of low amplitude of

accommodation for the right and left eyes respectively (516 and 533) was found in the

mainstream group compared to 291 and 288 in the learning disabled group The

123

relationships between the mainstream group and reduced amplitudes of accommodation of

the right (p=004) and left eyes (p = 0001) were found to be statistically significant

(plt005) Furthermore the relationship between the mainstream group and reduced ampli-

tude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to be moderate and

that between the mainstream group and the right eyes (Cramerrsquos = 0286) to be low

When accommodation amplitude is assessed monocularly it measures the response for each

eye individually and the limiting factor is the magnitude of blur-driven accommodation

Monocular amplitude measures are particularly important to determine whether a patient

has accommodative insufficiency 107 The amplitude measured binocularly is usually

greater than the monocularly measured amplitude due to the influence of both the blur-

driven accommodative response and accommodative and vergence responses 120 175

Clinically the amplitude of accommodation is considered important mainly when it falls

below the expected age norm in which case the child may experience blur vision at near

Secondly a difference of up to 2D between the two eyes may also be considered clinically

significant 120

An inefficient accommodation function may lead to difficulties in learning as the focusing

system of the eyes play a major role in the learning process Children who suffer some

anomalies of accommodation are more prone to fatigue quickly and become inattentive than

those who have norrmal accommodation function 87 Symptoms of accommodation

insufficiency are specifically related to near vision work 176

Factors affecting the measurement of amplitude include differences when measurements

are taken monocularly as compared to when assessed binocularly the angle of gaze target

size age refractive error race and climate 175

c Accommodation Posture During near vision the eyes are not usually precisely focussed

on the object of regard but the accommodation lags a small amount behind the target If

the accommodation lag is small then the blur it causes is insignificant if high then it can

result in blurred print during reading 142

124

About 3928 (11 subjects) of dyslexic subjects had lag of accommodation compared to

4193 (13 subjects) of the control group The mean accommodation lag was similar in

both groups 092 for control and 088 for the dyslexic group There were no statistically

significant differences between the two groups right eye (p = 083) and left eye (p = 060)

Similarly Evans at al 100 reported that the mean accommodation lag did not show any

statistically significant difference between the dyslexic and control groups On the

contrary Metsing and Ferreira 103 found a high prevalence of lead of accommodation for

the mainstream group but a high prevalence of lag of accommodation in the learning

disabled group The relationships were found to be statistically significant (plt005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000) In another

study Evans et al 91 reported a mean accommodation lag of + 112DS

An individual with a lag of accommodation habitually under accommodates and may lead

to difficulty with reading An accommodative response that manifests as an excessive lag

of accommodation may indicate latent hyperopia esophoria or may be associated with

accommodative insufficiency or accommodative spasm poor negative vergences or when

patient is overminused 177 The prevalence of high lag may be related to the prevalence of

latent hyperopia and esophoria in the present study

C Relative Accommodation (PRA NRA) The relative accommodation tests assess the

patientsrsquo ability to increase and decrease accommodation under binocular conditions when

the total convergence demand is constant67 It is also an indirect assessment of the

vergence system since the vergence demand remains constant while accommodative

demand varies 121 The results for the relative accommodation for all subjects from both

dyslexic and control groups were unexpectedly high The norm for the mean NRA and PRA

were given as approximately +2 or - 2 diopters from two reports consulted 119 121 while the

relative accommodation results in this study ranged between 3 and - 6 diopters Similarly a

higher relative accommodative value was reported by Chen et aland Abidin139 in a study of

vergence problems in Malay school children The only available study accessed that

125

reported on relative accommodation was conducted by lvarezand Puell 88 who reported

that the negative and positive relative accommodation values were similar in both groups of

children Statistically the results were (NRA Control group 19 06 poor readers 19

06 PRA Control group 23 09 Poor readers 20 13) compared to the findings from

the present study (NRA Control group was 311 047 NRA dyslexic group 322 079)

(PRA Control ndash 606 063 PRA dyslexic was -623 117) The p values were not

indicated

The higher values found in the present study may be due to the process of addition of trial

lenses specifically The phoropter has been reported to be a better technique to assess

relative accommodation 119

Although the relative accommodation was not assessed by many authors different opinions

were expressed regarding the assessment of relative accommodation Latvala et al 3 noted

that positive and negative relative accommodation ranges are ldquodifficult to take and

unreliablerdquo According to Morgan 178 the PRA and NRA results are often unusually high

but recommended that such tests results can be used as a sort of indicative way and can only

be interpreted loosely and suggested that the use should be restricted to special

circumstances Garcia and Fransisco 122 reported that high NRA can be associated with

disorders such as accommodative insufficiency and convergence excess while high values

of PRA are related to anomalies in which accommodative excess appears but concluded that

PRA and NRA findings were used mainly as complementary diagnostics tests of some

disorders in literatures consulted

A high value of the negative relative accommodation could indicate that the children were

exerting maximum accommodation effort Because 250D of accommodation is exerted at

the 40cm working distance the maximum amount of accommodation that would be

expected to relax accommodation at 40cm would be 250D A value greater than 250D

would mean that accommodation may not have been fully relaxed during the subjective

refraction 114 Generally a high value of the NRA may also mean that the refraction may

126

have been under corrected for hyperopia or over corrected for myopia 122 Garciaand

Francisco122 reported the relationship between high NRA values and under corrected

hyperopia to be due to the etiology of hyperopia

According to Grosvenor 114 the amplitude of accommodation is a limiting factor for the

expected value of the PRA For example ldquoIf a patient has an amplitude of accommodation

of only 150D we would expect the minus-lens-to-blur test to be no greater than -150Dhelliprdquo

A high value of the positive relative accommodation is related to convergence insufficiency 122 Illumination and the range of fusional vergence also affect the relative accommodation

An excessive illumination will give an erroneously high finding due to the increased depth

of focus 100

Dysfunctions of accommodation can significantly interfere with the comfort clarity speed

and accuracy of reading as the child develops reading skills 51 Wick and Hall 120 suggested

that the results for the tests of accommodation are more meaningful when analyzed

together as the results of individual accommodation function may not give a true reflection

of the childrsquos accommodation dysfunction

526 Fusional Reserves

Fusional vergence amplitudes is a clinical measurement of a person`s fusional vergence ability It

reflects the ability of the oculomotor system to maintain sensory fusion in spite of varying

vergence requirements 179 The horizontal vergence reserves describe the amount by which the

eyes can be converged or diverged whilst the subject maintains fusion 100161 It is a measure of

how much fusional vergence is available in reserve that can be used to overcome a phoria The

amount of base out prism required to produce diplopia is called positive fusional reserves

(measures convergence) 100 The measurements of convergence fusion are also referred to as the

lsquoamplitude of fusional reserves 111 Clinically the fusional amplitude provides information about

a patientrsquos ability to maintain comfortable binocular vision 100112

The participants from both the dyslexic and control groups either could not report or understand

127

blur so the result for break and recovery was used in all analysis of vergence function Evans et

al 100 reported similar observations For the fusional reserves only the findings for the near

positive fusional vergence (convergence) are emphasized The positive fusional

reservesvergence (measures convergence ability) may be more important in assessing reading

dysfunction More so it has been suggested by other authors 112121180 that near measurements

are more relevant in assessing vision functions in dyslexic children

Based on Bishops 111 recommended norm of between 30 and 40 prism diopters for positive

fusional amplitude at near the prevalence of abnormal positive fusional vergence at near was

83 for the control and 74 for the dyslexics This appears unusually high Walters 168

explained that a possible reason for an unexpected high base out finding is that exophoric

conditions makes compensating for base out more difficult than base in testing It is unclear if

this is the case in the present study although the prevalence of exophoria at near in the present

study was 95 The compensation for a phoria is maintained by fusion reflex so knowing the

magnitude of the vergence amplitude that is needed to compensate a given phoria is very useful

clinically181

For the present study at near the negative fusional vergence (base in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base out

vergences) was similar for both groups Overall there was no statistically significant difference

between the groups (p = 021 for BI break at near) and (p = 084 for BO break at near) Evans et

al 100 reported that the dyslexics have reduced fusional reserves compared to the control group

Bucci et al 89 reported reduced divergence capabilities in dyslexics at near

When assessing fusional reserves the blur point is a function of the flexibility between vergence

and accommodation When little flexibility exists the addition of even low prism causes a

simultaneous accommodation shift with resultant blur Lack of low blur findings may indicate an

inadequate flexibility bond between accommodation and convergence The break point is an

indication of the quality of binocular function Break values will be adequate when fusion ability

is good while the recovery measure is a more subtle indicator of the quality of binocular function

Consequently reduced blur break or recovery findings indicate the presence of near point stress

128

182 Some subjects from both groups had break values higher than 40 prism diopters for base in

and base out reserves Wesson et al 125 in their study reported a similar observation on objective

testing of vergence ranges Such high break findings may be because suppression was not

controlled while assessing the fusional reserves in the present study When suppression is

controlled the average vergence values will be lower because the test is stopped when the

suppression is detected If suppression is not monitored the break is not detected until the

stimulus is outside the suppression zone and a higher vergence value is obtained 125 However it

is important to note that the significance of such suppression in binocular individuals is unknown 126 Again in special cases such as suppression large abnormal findings (compared to

normative ranges) may permit better functions in reading than a lower abnormal finding 183 For

example an individual who totally suppresses the vision of one eye is less apt to have difficulty

at reading than the individual who only partially suppresses the vision of one eye 183

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups The

mean fusional convergence capacities at distance were 1680 prism dopters for both the control

and the dyslexic group At near the corresponding figures were 2640 prism diopters and 2670

prism diopters respectively The mean fusional divergence capacity at distance was 650 prism

diopeters and 620 prism diopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 1050 and 1020 prism diopters respectively There was no

statistically significant difference between the groups

Latvala et al 3 reported a prevalence of fusional amplitude greater than or equal to 32 prism

diopters to be 61 for the control group and 75 for the dyslexic group using referral criteria

of 32 prism diopters at a distance of 33cm At near a testing distance of 40cm was used in this

study The difference in prevalence between the two studies might be from the different test

distances used Furthermore as detailed by Rouse et al 158 large intra subject intra-examiner

and inter-examiner variations in the assessment of fusional reserves have been reported

Vergence amplitudes have been reported to vary with alertness that is whether the subject is

tired or rested or under the influence of a toxic agent 184

129

Bedwell et al 171 reported that neither convergence nor divergence were significantly related to

reading difficultyrdquo Similarly Metsing and Ferreira103 found no statistically significant

difference between fusional vergences amplitude (Base-out base-in) at near No details were

provided

There are several limitations with the assessment of fusional reserves Sheedy (cited by Rouse et

al) 158 remarked that ldquo the general opinion is that when fusional vergence tests are repeated on

the same patient the second value found may be quite different from the first a difference of 10

PD from one fusional vergence amplitude measurement to another is not unusual unless rigorous

controls ar a typical difference of 3- 4 prism diopters but can measure differences as large as 12

prism diopter on follow up visit and that inter-examiner variation can be as large as 10 to 16

prism diopters The authors concluded that the positive fusional vergences difference could be

due to children having difficulty understanding the instructions or expected endpoints children

being slower responders or may be poorer observers The blur readings on the vergence reserves

for both distance and near were excluded in the analysis as it was difficult to get the children to

elicit a proper response It is possible that the children did not understand the instruction or the

concept of blur despite repeated explanations and trial readings This variable is often difficult

for young subjects to understand Similar problems were encountered by Evans et al 100 who

commented that several subjects were unable to appreciate a blurrdquo Scheiman et al 126 stated

that ldquowe were unable to consistently obtain a blur finding from our samplerdquo and that rdquo less

than13 of the subjects were able to report a blur we therefore only recorded the break and

recovery findingsrdquo

Wesson et al125 emphasized that the ldquomajor limitations of any vergence measurement

isrdquohellipnecessity to obtain a subjective response to blur break and recovery and that working with

children whose responses are not reliable is difficultrdquo It was suggested that a break and recovery

point could be determined by observing the subjects eye movements as prism power is increased

This technique was applied in this study However most children have difficulty maintaining

fixation long enough to measure vergence ranges

Vergence is influenced by several factors including awareness of the distance of the object

130

(proximal vergence)cross-linking with the accommodative system (acommodative vergence) and

the fine tuning of ocular alignment during the fusion of each monocular image into a single

percept (fusional vergence) 161 and other factors such as test target and lighting conditions 125

Generally in the assessment of a patientrsquos binocular status the fusional reserves may be more

useful when analyzed with the phoria measurements 115

The vergence system is closely related to the accommodative system and symptoms may at times

appear similar The symptoms associated with deficiencies with the vergence system include

letters or words appear to float or move around postural changes noted when working at a desk

difficulty aligning columns of numbers intermittent diplopia at either distance or near 185

Furthermore ldquodysfunctional vergence could cause an excess of eye movements especially of

small saccades The ability to continue the rapid decoding of the visual characters decreases as

more stress is placed on the vergence mechanism of the dyslexic during reading which makes

them tire more quickly than normal readersrdquo 6

A possible explanation for the relationship between vergence control and reading proposed by

Riddel et al (cited by Morad et al) 186 is that children with poor vergence control had impaired

accuracy of spatial localization that may impend their ability to accurately determine the position

of letters within words

527 Ocular Pathology

Two participants from the dyslexic group had cataracts Only the study by Hoffamn 83 reported

that one child had conjunctivitis

54 Summary

The results of this study do not vary significantly from the international studies in spite of the

differences in study designs tools and sample sizes Although studies have indicated differences

131

in the prevalance of open angle glaucoma and myopia between Caucasian and African

populations this study does not indicate a specific difference in the prevalance of vision variables

between Caucasian and African dyslexic school children However the lack of studies on

African children makes it difficult to reach any conclusions regarding structural differences that

may affect the presence of dyslexia Comparing the findings of this study with those done

internationally highlights the complexity of investigating the visual conditions that may be

associated with dyslexia and the need for rigorous and consistent testing methods in order to be

able to compare results

Having completed a discussion of the study findings in Chapter Five Chapter Six will present the

study conclusion and indicate its limitation as well as recommendations for future studies

132

CHAPTER SIX CONCLUSION

61 Introduction

Being able to read and write is an essential part of being able to interact with the world As

vision plays a major role in reading and the learning process a normally working visual system is

essential for efficient reading

With as many as 20 of Caucasian children being affected to a greater or lesser degree by

dyslexia 1631 there is a need to identify its cause and develop remedial actions to reduce its

effects As studies 4 59 have shown that intellectual ability is not the cause of dyslexia research

to enable people to reach their full potential in spite of their impaired reading ability requires a

multidisciplinary approach of which optometrists are part

This study aimed to determine the prevalence of vision conditions in a South African population

of African dyslexic children and to study the relationship between vision and dyslexia in an

African setting This was done by investigating visual acuity refraction and binocular functions

between two groups of 31 African school children one group attending a school for children with

learning difficulties and the other a mainstream school This enabled a study of possible vision

defects in African dyslexic children as well as a comparison between the two groups

The study targeted African school children its objectives being to

1 determine the distribution of visual acuity disorders among dyslexic children

2 determine the distribution of refractive errors among dyslexic children

3 determine the distribution of heterophorias among dyslexic children

4 determine the distribution of strabismus among dyslexic children

5 determine the distribution of accommodation disorders among dyslexic children

6 determine the distribution of vergence disorders among dyslexic children

7 determine the distribution of ocular pathology among dyslexic population

133

8 compare these findings to a similar group of non-dyslexic children

A review of the literature showed that most studies that investigated visual functions in dyslexic

school children were conducted on Caucasian population and that the findings of these studies

were inconclusive The broad areas of vision investigated in the previous studies 2-6 87-89 were

visual acuity and refraction binocular vision and ocular pathology

As studies conducted on the African population was lacking this study examined visual acuity

(using LogMar charts) refraction (static retinoscopy without cycloplegia) near point of

convergence (RAF rule) [accommodation functions 2 flipper lenses to assess accommodation

facility MEM retinoscopy for accommodation posture and pushndashup method to evaluate

amplitude of accommodation] ocular alignment (cover test and Maddox Wing) and strabismus

(Hirschberg test) and fusional reserves (using prism bars)

The study was conducted on a African population of 31 dyslexic school children selected from a

school for children with learning difficulties and 31 control participants from a mainstream

school in Durban Their ages ranged between 10-15 years The participants were selected using

the convenient sampling method as there were only a few participants classified as being

dyslexics All data collection procedures were conducted at the respective schools

The prevalence of vision conditions were presented in percentages () Data was analyzed using

the Statistical Package for Social Sciences (SPSS) and was expressed as mean (M) and standard

deviation (SD) and calculated for descriptive purposes at 95 confidence interval (CI) The level

of significance considered to support a hypothesis was taken as P lt 005 For comparison all

data from both groups was subjected to a two sample t test (2-tailed)

134

62 Summary of findingsIf an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants (control group) would be

expected

621 Visual Acuity

The prevalence of visual acuity worse than 69 (23 dyslexic and 225 control) is the same in

both the dyslexic and the control group and there was no statistically significant difference (p =029 023 right and left eye respectively This is similar to the findings by Buzzelli 6 and

Goulandris et al 90 which found no statistically significant difference between the two groups

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexia and the control group (p=029 for the right eye and 023 for the left eye)

622 Refractive Errors

The prevalence of refractive errors was similar in both groups There was no statistically

significant difference between the two groups (p=066 for the right eye and 092 for the left eye)

This is similar to the findings by Evans et al 2 which found no statistically significant difference

between the two groups (p=058)

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 066 for the right eye and 092 for the left eye)

a Hyperopia Hyperopia was more prevalent in the dyslexic group than in the control group

This is consistent with findings in other studies 88 93 95

b Myopia The prevalence of myopia was the same in both groups This is similar to the

findings by Eames 95 which found no difference in the prevalence of myopia between the

two groups

135

c Astigmatism Astigmatism was more prevalent in the control group than in the dyslexic

group This is similar to the findings by Alvarez et al 88 which found a higher prevalence of

astigmatism in the control group than in the dyslexic group

d Anisometropia The prevalence of anisometropia was the same in both groups (2 subjects

each) This is comparable to results reported by Latvala et al 3 where the prevalence of

anisometropia was 36 (2 subjects) for the dyslexic group and 6 (3 subjects) for the

control group

e Amblyopia Amblyopia was more prevalent in the dyslexic group compared to the control

group This is similar to the findings by Latvala et al 3 and Rosner and Rosner 93 which

found a higher prevalence of astigmatism in the dyslexic than in the control group

623 Near Point of Convergence

The prevalence of remote NPC (ge10cm) was higher in the control group than in the dyslexic

group and there was a statistically significant difference between the two groups (p= 0049) This

is similar to the findings by Kapoula et al 87 Latvala et al 3 and Evans et al 91 who found a

statistically significant difference between the dyslexic and the control groups on the NPC

functions

The null hypothesis was rejected as there was a statistically significant difference between the

dyslexic and control groups (p = 0049)

624 Heterophoria

The prevalence of exophoria at near was higher in the dyslexic group (95) than in the control

group with no exophoria There was no statistically significant difference between the two

groups (p= 059) The prevalence of esophoria in the dyslexic group was higher than that found

136

in the control group and there was no statistically significant difference (p =046) This is similar

to findings in other studies 3 12101163 which found no statistically significant difference between

the two groups

The null hypothesis was accepted as there was no statistically significant difference between the

dyslexic and control groups (p=059 for exophoria) and (p=046 for esophoria)

625 Accommodation Functions

a Amplitude of Accommodation The dyslexic group had a reduced monocular amplitude

of accommodation compared to the control group but there was no statistically significant

difference between the two groups (right eye p = 007 left eye p=022) This is similar

to the findings reported in other studies 3 87 91102 which found no statistically significant

difference between the two groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation amplitude (right eye

p=007 left eye p=022)

b Accommodation Facility The participants from the dyslexic group performed

significantly worse than the control group in accommodative facility function and there

was a significant difference between the two groups (p=0027) This result corroborates

findings reported in other studies 12 88 98100 which found a statistically significant

difference between the two groups

The null hypothesis was rejected as there was a statistically significant difference between

the dyslexic and control group in accommodation facility (p=0027)

c Accommodation Lag The prevalence of lag of accommodation was higher in the control

group compared to the dyslexic group and there was no statistically significant difference

There were no statistically significant difference between the two groups (right eye p=083

137

and left eye p= 060) This is similar to the findings by Evans et al 100 which found no

statistically significant difference between the two groups The mean accommodation lag in

the present study were 092D for control and 088D for the dyslexic which is comparable to

reports by Evans et al 91 who reported a mean accommodation lag of + 112D

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation posture

d Relative Accommodation (PRA NRA) The mean values for the relative

accommodation are similar in both groups and there was no statistically significant

difference between the groups (p=068 for NRA and p =051 for PRA) This is similar

to the findings reported Alvarez and Puell 88 which found no statistically significant

difference between the two groups in relative accommodation

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups in relative accommodation

626 Fusional Reserves

For the present study at near the negative fusional vergence (base-in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base-out

vergences) was similar for both groups There was no statistically significant difference between

the groups (p= 021 for BI break at near) and (p = 084 for BO break at near) This is similar to

the findings by Evans et al100 which found no statistically significant difference between the two

groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 021 for BI break at near) and (p = 084 for BO break at near)

627 Ocular Pathology

138

Two participants from the dyslexic group had cataracts

63 Implications of the Study Findings

a Causal Relationship This study presented the prevalence of some vision defects in a

South African African population of dyslexic school children as compared to a population

of non-dyslexic children and could not establish any causal relationship between vision and

dyslexia The vision parameters that showed that statistical significance may only be

reflective of factors associated with dyslexia but in a non-causal way This may mean that

the prevalence of the particular visual variable is much higher in the dyslexic population

than in the control group Vision is essential for reading but the vision anomalies that are

found to be associated with dyslexia also occur in children who do not have reading

difficulties It is therefore possible that other complex vision functions may be more related

to the etiology of dyslexia than peripheral visual factors such as NPC and accommodative

facility which showed statistically significant differences in the present study

b Risk Identification The findings of this study also suggest that dyslexic children are not

at more risk for the identified vision condition compared to non-dyslexic children

However even when vision defects may not be the cause of dyslexia the presence of

uncompensated vision defect in a dyslexic child may constitute difficulty while reading

and should be compensated for Improvement in vision will make reading easier for the

dyslexic child

64 The Study Strengths

The study methodology was based on reports found in other studies and consisted of the

following

a Classification The subjects selected for the dyslexic group represent a dyslexic population

that was classified according to the criteria used in the study by Latvala et al 3 and Evans et

139

al 100 The study was undertaken at a school for learning disabled learners and all students

were assessed and categorized as being dyslexic by the psychologists

b Study Examiner Examination of the subjects was conducted by the same optometrist who

had some experience in paediatric optometry This minimised the possibility of bias and

inter-examiner variability

c Examination Techniques The examination techniques used in this study were the same

as the techniques used in major referenced studies which were published in peer-reviewed

journals

d International Relevance It is the first study that has assessed broad vision functions in

a dyslexic population in an African setting

6 5 Limitations of the Study

The factors which may limit the generalization of findings of the study are as follows

a Inattentiveness This occurred in some children after numerous tests and the possibilities

of their lack of understanding of certain instructions However this problem was

minimized as trial readings were taken for some procedures and the participants were

instructed accordingly As stated in Section 34 testing was discontinued when the child

was tired This approach helped to minimize fatigue which could have negatively

affected the results In addition objective techniques that required minimal responses

from the participants were utilized in several procedures such as the cover test

retinoscopy and the monocular estimation method for evaluating accommodation posture

b Convenience Sampling This method of sampling was used given the limited number of

available subjects from the target population The available studies 235687-91102103 on

the subject of dyslexia learning disabilities and vision also used the convenience

sampling method

140

c Generalised Findings The generalization of the findings of this study is limited by the

fact that the refraction data was collected without the use of cycloplegia At the inception

of the data collection the diagnostic drug usage by optometrists was not approved in the

scope of practice in South Africa

d Sample Size The sample size was small as there was only one school for dyslexic

African children in Durban at the time of this study However review of the literature

also reveals that the sample sizes on studies conducted on dyslexia learning disabilities

and vision were typically small The targeted sample size of one hundred for this study

could not be met due to the limited number of subjects available The average sample size

of eight published studies 356 8790100 102 conducted on dyslexic children was 41

e Suppression Control The assessment of binocular accommodation facility and the

vergence reserves were performed without controlling for suppression

f Assessment of Relative Accommodation the use of a Phoropter would have yielded a

more conclusive result

g Eye Movements these were not assessed

Despite the acknowledged limitations inherent in this study the study provides useful

information and a research perspective on the prevalence of vision defects in a South African

population of dyslexic children which has not been conducted before

66 Recommendations

The following recommendations are made based on the findings of this study

661 Future Research

141

1 The same study with a similar protocol larger sample size with randomized sampling

2 Investigate the relative accommodation in in both groups using the phoropter

3 Assess the binocular functions with suppression control

4 Assess refractive error under cycloplgia

662 General Recommendations

1 All dyslexic children should have their eyes examined routinely to rule out

possibility of vision defects impacting on their reading performance

2 The Department of Education be reminded that visual defects can impact negatively on

the reading ability of learners and that teachers need to be more aware of pupils who may

present with signs of visual problems such as holding book too close or battling to see

the chalk board

3 The principals of the mainstream school should also be informed that vision defects were

detected in the non-dyslexic children

67 Conclusion

As a neurological condition which manifests primarily as a language-based disorder with

difficulty with words dyslexia requires a multi-disciplinary approach to its management with

an eye examination being needed to eliminate any possible effects that ocular conditions may

contribute to reading problems

The study provided the prevalence of disorders of visual acuity and refractive errors near point

of convergence accommodation dysfunction heterophoria strabismus and fusional reserves

among African dyslexic children of age range 10-15 years The only vision variable that was

more prevalent and that is significantly associated with dyslexia was the binocular

accommodation facility while only the near point of convergence (break and recovery) was

significantly more prevalent in the control than the dyslexic group The existence of these

statistically significant differences between the two groups may not imply clinical relevance

due to the small sample size The comparison of vision characteristics between the dyslexic

142

and control group indicates that the dyslexic children are not more at risk of having these

vision condition than the non-dyslexic children although sample size was small

Further research on African children will add to the body of knowledge about this group with

respect to the relationship between vision conditions and dyslexia about which very little is

known Only a few studies have provided evidence of the extent of dyslexia among African

children yet even a small percentage could result in many millions of children being affected

given the population of Africa While the problems that result from dyslexia are not life

threatening they do affect peoplersquos opportunities and quality of life

Unfortunately many African countries do not have the resources to provide specialized

schools for children with learning disabilities and every effort therefore needs to be made to

correctly identify the problems and possible causes of dyslexia In order for African countries

to implement the strategies to meet their millennium development education goals of ensuring

that by 2015 all children will be able to complete a full course of primary schooling they

need to provide not only for children in mainstream schools They need also to plan for

children whose academic ability would be enhanced by the appropriate diagnosis of learning

difficulties the provision of a simple eye test and spectacles or corrective apparatus where

appropriate

Dyslexia is not a disease for which a cure can be found rather long-term sustainable

interventions need to be put in place to ensure that the problems are identified and appropriate

remedial action is taken to minimize the impact it has on their life While many African

countries may not be able to afford schools for children with learning disabilities greater

awareness of the manifestation of the condition needs to be made to all teachers in an effort to

provide these children with the possibility of staying in main stream schools for as long as

possible where there are no alternatives With this in mind it is anticipated that this study will

contribute to the body of knowledge on vision conditions of African dyslexic school children

143

REFERENCES

1 Scheiman M Rouse M 1994 Optometric Management of Learning -Related Vision Problems Missouri Mosby Ist Edition

2 Ygge J Lennerstarnd G Axelsson I Rydberg A Visual Function in a Swedish Population of Dyslexic and Normally Reading Children Acta Ophtahlmol(Copenh)1993 71 (1) 1-9

3 Latvala ML Korhoenen TT Penttinen M Ophthalmic Findings in Dyslexic School Children Br J Ophthal 1994 78 339-343

4 Aasved H Ophthalmological Status of School Children with Dyslexia Eye 1987 161-68

5 Evans BJ Drasdo N Richards IL Investigation of some Sensory and Refractive Visual Factors in Dyslexia Vision Research 1994 34 (14) 1913-1926

6 Buzzelli AR Stereopsis Accommodative and Vergence Facility Do they relate to Dyslexia Optom Vis Sc 1991 68 (11) 842-846

7 Naidoo KS Africa Vision Research Institute Monograph 2007

8 Wu SY Nemesure B Leske MC Refractive Errors in a African Adult Population The Barbados Eye Study Invest Ophthal amp Vis Sci 1999 40 (10) 2179-2184

9 Tielsch JM Sommer A Katz J The Prevalance of Refractive errors Among Adults in the United States Western Europe and Australia Arch Ophthalmology 2004 122 495-506

10 Rudnicka AR Shahrul M Owen C Variations in Primary Open-Angle Glaucoma Prevalence by Age Gender and Race A Bayesian Meta-Analysis Invest Ophth Vis Sc 2006 47 (10) 4254 - 4265

11 Raju D 1997 Poor Oculomotor Control in Dyslexia Masters Degree Thesis University of Durban-Westville

12 Keily PM Crewther SG Crewther DP Is there an Association between Functional Vision and Learning Clin amp Exp Optom 2001 84 (6) 346-353

13 Krupska M Klein C 1995 Demystifying Dyslexia London Language and Literacy Unit

14 Schumacher J Hoffmann P Schma C Schulte G Genetics of Dyslexia The Evolving Landscape J Med Genet 2007 44 289ndash297

144

15 Pushpa S Nallur B Biological basis of Dyslexia A Maturing Perspective Current Science 2006 90 (2) 168-175

16 Sherman G International Dyslexia Association Fact Sheet 969 January 2000

17 Berninger V Dyslexia The Invisible Treatable Disorder Learning Disability Quarterly2000 23 (3) 125

18 Harrie RP Weller C Dyslexia httpwwwericecorg Last accessed January 2007

19 Critchley M Is Developmental Dyslexia the Expression of a Minor Cerebral Damage The William Copeland Memorial Lecture National Hospital London 1966

20 Christenson GN Griffin JR Wesson MD Optometrys Role in Learning Disabilities Resolving the Controversy J of Am Optom Assoc 1990 61 (5) 363-372

21 Evans B 2007 Dyslexia and Vision 2nd Edition Whirr Publishers London

22 Flax N Visual function in Dyslexia Am J Optom Arch Acad Optom 1968 45 574-87

23 Optometric Clinical Practice Guidelines Care of the Patient with Learning-Related Vision Problems America Optometric Association 2000

24 Tan L Spinks JA Eden GF Perfetti CA Siok WT Reading depends on Writing in Chinese Pro Natl Acad Sc 2005 102 (24) 8781 ndash 8785

25 httpfacultywashingtonedu What Is Dyslexia Last Accessed 15th September 2008

26 Shaywitz S Overcoming Dyslexia A new and complete science-based program for reading problems at any level 2003 New York NY Alfred A Knopf

27 Holland KC A look at the Reading Process Optician July 3rd 1987 11-13

28 Bedwell CH The Visual and Ocular aspects of Reading Difficulty Optician Sept 24th 1982 26-38

29 Rayner Keith Eye Movement in Reading Perceptual and Cognitive ProcessesCanadian Psychology Feb 1995

30 Sibylina M Brochure from Learning Disabilities Council of America 1998

31 Pavlidis GT Eye Movements The Diagnostic Key to Dyslexia S Afr OptomJuneAugust 1988 29-39

32 Evans B The Role of the Optometrist in Dyslexia Part 1 Specific Learning Difficulties Optometry Today 2004 January 30 29-33

ii

DECLARATION

I hereby declare that this thesis is my own original work It is submitted to the University

of KwaZulu-Natal in partial fulfilment for the degree of Master of Optometry

SAMUEL OTABOR WAJUIHIAN

SIGNATURE helliphelliphelliphelliphelliphelliphelliphelliphelliphellip

DATE APRIL 2010

iii

ACKNOWLEDGEMENTS

There are at times that this project seemed to be a lsquonever ending journeyrsquo and many people have encouraged me greatly I wish to express my profound gratitude and acknowledgement of the contributions of the following people who have given me support in different ways

Professor Kovin Naidoo (my supervisor) for his guidance throughout the entire research process

The examiners for their critical evaluation of the study

My family for their patience throughout the time I was not there for them Special thanks to my wife Joyce for all her encouragements right from the beginning of the project To my parents for giving me the discipline never to give up no matter how rough things may be

Carrin Martin for her invaluable support in editing this thesis Members of staff of the International Center for Eye Care Education Sandro Thaver for the statistical analysis Mirashnee Rajah and Diane Wallace for their continued administrative and moral support

All members of staff of the Optometry Department University of Kwazulu Natal for their assistance in different ways

Celia G of ILAMO for assistance with referenced articles

Department of Education for granting permission to have access to the schools

The Principals Parents and Learners of schools (Khulangolwazi special school and Addington Primary schools) Durban for their cooperation

Authors of the numerous studies I consulted whose studies upon which the foundation of this study was based

The University of KwaZulu-Natal for initial financial support

iv

Abstract

Dyslexia is a neurological disorder with genetic origin that affects a personrsquos word

processing ability their spelling writing comprehension and reading and results in poor

academic performance As a result optometrists are consulted for assistance with the

diagnosis and treatment of a possible vision condition Optometrists are able to assist with

treatment as part of a multidisciplinary management approach where optometric support is

necessary International studies have indicated that up to 20 of Caucasian school children

are affected by dyslexia while there are no similar figures for African children Studies have

been done to assess the extent of visual defects among Caucasian dyslexics but not among

African dyslexic children The aim of the study is therefore to determine the prevalence of

vision conditions in an African South African population of dyslexic school children and to

investigate the relationship between dyslexia and vision

The possible relationship between dyslexia and vision conditions has been recognized as an

important area of study resulting in research being conducted in many countries Studies

have been undertaken by optometrists and ophthalmologists who differ in their approach

and attitude on how vision conditions affect dyslexia A review of the literature revealed

three broad areas of vision that may impact on reading ability these being acuity defects

binocular vision and ocular pathology Acuity defects consist of visual acuity and refractive

error Areas of binocular vision evaluated in the literature include near point convergence

heterophoria strabismus accommodative functions vergence facility and reserves

Hyperopia was the only vision variable that was found to be consistently associated with

difficulties with reading but not causally while findings on other variables were

inconclusive However all the studies acknowledged the complexity of the condition and

the need for a comprehensive multidisciplinary management approach for its diagnosis and

management

The study was undertaken in the city of Durban South Africa using a case-control study of

two groups of African school children between the ages of 10 and 15 Both study groups

consisted of 31 children of normal intelligence who were matched in gender race and

v

socio-economic status The case group attended a school for children with learning

disabilities while the control group attended a mainstream school At the time of the study

only one school catered for African children with learning disabilities and only 31 of its

pupils were diagnosed with dyslexia Ethical approval was obtained from the University of

KwaZulu-Natal permission to undertake the study in the identified schools was obtained

from the Department of Education and the school principals consented on behalf of the

learners as it was not always possible to reach the individual parent

The researcher (an optometrist) visited both schools by appointment where rooms were

made available to do the testing and the tests were explained to all participants The

LogMar Acuity Charts were used to assess visual acuity and static retinoscopy was used to

assess refractive error Binocular vision was tested using the cover test for ocular alignment

the Hirschberg test for strabismus RAF rule for near point of convergence 2 D flipper

lenses for accommodation facilities Donderrsquos push up methods using the RAF rule for

amplitude of accommodation plus and minus lenses for relative accommodation monocular

estimation technique for accommodation posture and prism bars for vergence reserves

Ocular pathology was assessed using a direct ophthalmoscope

The dyslexic group presented with the following Refractive errors hyperopia 65

myopia 65 astigmatism 10 anisometropia 65 remote near point of convergence

33 esophoria at near 3 exophoria at near 95 accommodative infacility 54 and lag

of accommodation 3928 The dyslexic group had relatively reduced fusional reserve

compared to the control group

The control group presented with the following Refractive errors hyperopia 3

astigmatism 13 anisometropia 65 remote near point of convergence 48 esophoria at

near 0 exophoria at near 0 accommodative infacility 33 and lag of accommodation

4193

The prevalence of a remote NPC was higher in the control group than in the dyslexic group

and there was a statistically significant difference between the two groups NPC break

vi

(p=0049) and recovery (p=0046) The prevalence of poor binocular accommodation

facility at near was higher in the dyslexic group than in the control group and there was a

statistically significant difference between the two groups (p = 0027)

Vision defects such as hyperopia astigmatism accommodation lag convergence

insufficiency poor near point of convergence and accommodative infacility were present in

the dyslexic pupils but they were no more at risk of any particular vision condition than the

control group This study provided the prevalence of vision conditions in a population of

African dyslexic children in South Africa the only vision variable that was significantly

more prevalent in the dyslexic population being the binocular accommodation facility at

near although the study was unable to find a relationship between dyslexia and vision The

statistically significant difference may not imply clinical significance due to the small

sample size However it is recommended that any vision defects detected should be

appropriately compensated for as defective vision can make reading more difficult for the

dyslexic child

The sample size may have been a limitation however this was comparable with studies

reviewed most of which had sample sizes of less than 41 Due to the range of possible

ocular conditions that could affect dyslexia it is recommended that a larger sample size be

used to ensure more conclusive results Testing for relative accommodation with a

phoropter would provide more accurate results and accommodation facility and fusional

reserves would be better assessed with suppression control The study provides information

and an indication of research needs regarding the prevalence of vision defects in an African

South African population of dyslexic children

vii

TABLE OF CONTENTS

CONTENTS PAGETITLE PAGE iDECLARATION iiACKNOWLEDGEMENTS iiiABSTRACT ivTABLE OF CONTENTS viiLIST OF APPENDICES xiLIST OF TABLES xiiLIST OF FIGURES xiiiLIST OF PUBLICATIONS FROM THIS THESIS xiv

CHAPTER ONE INTRODUCTION 1

11 Background and Rationale for the Study 112 Aim 213 Objective 214 Research Questions 215 Significance of study 316 Goals of Study 317 Null Hypothesis 318 Research (alternative) Hypothesis 419 Scope of Study 4110 Assumptions 5111 Study Outline 5112 An overview of Dyslexia 5113 The Reading Process 8114 Eye Movement in reading 9115 Characteristics Features of Children with Dyslexia 10116 Implications of Dyslexia for learning 11

1161 Reading and Comprehension 121162 Handwriting 121163 Proof-reading 12

117 Prevalence of Dyslexia 12118 Aetiology of Dyslexia 13

1181 Neurological Factors 141182 Genetic Factors 151183 Peri-natal Factors 151184 Substance Use during Pregnancy 151185 Environmental Factors 161186 Nutrition 16

119 Diagnosis of Dyslexia 171191 Exclusionary Method 17

viii

1192 Indirect Method 171193 Direct Method 18

120 Functional Imaging Techniques used in the Diagnosis and Study of Dyslexia 191201 Positron Emission Tomgraphy (PET) 191202 Functional Magnet Resonance Imaging (fMRI) 19

121 Classification of Dyslexia 201211 Primary Dyslexia 211212 Secondary Dyslexia 21

122 Sub-types of Dyslexia 211221 Dysphonetic (auditoryphonetic) 211222 Dyseidetic (visual-spatial) 211223 Dysnemkinesia (motor) 211224 Dysphonetic-Dyseidetic 23

123 Theories of Dyslexia 231231 Phonological-Deficit Theory 231232 Cerebellar Theory 251233 Visual Deficit Theory 261234 Auditory Processing Theory 27

124 Cultural Differences in Dyslexia 28125 Treatment Intervention 30126 Role of Optometry in the Multidisciplinary Management of Dyslexia 30

1261 Case History 311262 Refractive and Binocluar Functions 311263 Visual-Motor Skills 311264 Visual-Spatial Analysis 321265 Short-term Visual Memory (visual-motor recall) 321266 Auditory Analysis Skills 32

127 Optometric Treatment Options 331271 Correction of Vision Defects 331272 The use of Coloured Overlay 33

128 Potential Mechanism for Benefit from Tinted Lenses 35129 Summary of Chapter One 37

CHAPTER TWO LITERATURE REVIEW 39

21 Introduction 3922 Visual Acuity 4023 Refractive Errors 47

231 Myopia 47232 Hyperopia 49233 Astigmatism 49234 Amblyopia 50235 Anisometropia 50

24 Near point of Convergence 53

ix

25 Heterophoria 5526 Strabismus 5827 Accommodation Functions 59

271 Amplitude of Accommodation 59272 Accommodation Facility 61273 Relative Accommodation 64274 Accommodation Posture 64

28 Vergence Facility 6529 Fusional Reserves 66210 Ocular Pathology 68211 Summary of Chapter Two 73

CHAPTER THREE METHODOLOGY 75

31 Introduction 7532 Research Design 75

321 Sampling Design 75322 Study population and Participants 75323 Inclusion Criteria for Dyslexic Group 76324 Exclusion Criteria for Dyslexic Group 76

33 Ethical Clearance 7734 Testing Protocol 7735 Study Setting 7736 Data Collection Technique 78

361 Visual Acuity 79362 Refractive Error 79363 Near Point of Convergence 80364 Strabismus 80365 Heterophoria 81366 Evaluation of Accommodation Functions 83367 Fusional Reserves 85368 Ocular Health 87369 Vergence Facility 87

37 Statistical Analysis 8738 Summary of Chapter Three 89

CHAPTER FOUR RESULTS 91

41 Introduction 9142 Prevalence of Vision Defects 93

421 Visual Acuity 91422 Refractive Errors 92423 Near Point of Convergence 94424 Ocular Health 96

x

425 Heterophoria 96426 Accommodation Functions 97427 Fusional Reserves 101

43 Summary of Chapter Four 105

CHAPTER FIVE DISCUSSION 107

51 Introduction 10752 Prevalence of Vision Defects 107

521 Visual Acuity 109522 Refractive Errors 116523 Heterophoria 116524 Near point of Convergence 118525 Accommodation Functions 120526 Fusional Reserves 126527 Ocular Pathology 130

53 Summary of Chapter Five 130

CHAPTER SIX CONCLUSION 132

61 Introduction 13262 Summary of Findings 134

621 Visual Acuity 134622 Refractive Errors 134623 Near Point of Convergence 135624 Heterophoria 135 625 Accommodation Functions 136 626 Fusional Reserves 137627 Ocular Pathology 137

63 Implications of the Study Findings 138 64 The Study Strengths 138 65 Limitations of the Study 139 66 Recommendations 140

661 Future Research 140662 General Recommendations 140

67 Conclusion 141

REFERENCES 143

xi

LIST OF APPENDICES PAGE

APPENDIX A Clarification of Terms 156APPENDIX B Request for Consent from Department of EducationAPPENDIX C Letter of Consent from Department of EducationAPPENDIX D Letter of Consent from Addington Primary schoolAPPENDIX E Visual Acuity Conversion Table

xii

LIST OF TABLES

TABLE TITLE PAGE

11 Characteristic Features of Dyslexia 1012 Features of the Magnocelular and Parvocellular systems 2721 Conclusions from Major Studies on Dyslexia and Vision 7031 Data Collection Tools and Measurement Outcomes 7832 The Diagnostic Criteria for each Test Variable 8741 Descriptive Statistics for Visual Acuity 9242 Descriptive Statistics for Refractive Error 9443 Descriptive Statistics for Near Point of Convergence Break

and Recovery Points 9544 Descriptive Statistics for Heterophoria 9745 Descriptive Statistics for Accommodative Posture 9846 Descriptive Statistics for Binocular Accommodation Facility 9947 Descriptive Statistics for Amplitude of Accommodation 10048 Descriptive Statistics for Relative Accommodation 10149 Descriptive Statistics for Base-in Vergence Reserves at Distance 102410 Descriptive Statistics for Base-in Vergence Reserves for Near 102411 Descriptive Statistics for Base-out Vergence Reserves at Distance 103412 Descriptive Statistics for Base-out Vergences at Near 103413 Descriptive Statistics for Dyslexic and Control Groups

for all Variables 104

xiii

LIST OF FIGURES

FIGURE TITLE PAGE

41 Prevalence of Visual Acuity 9242 Prevalence of Refractive Errors (total) 9343 Prevalence of the Types of Refractive Errors 9444 Prevalence of NPC Break 9645 Prevalence of Phoria at Near 9646 Prevalence of Accommodation Lag 9847 Prevalence of Reduced Binocular Accommodation Facility at Near 9948 Prevalence of Relative Accommodation 101

1

CHAPTER ONE INTRODUCTION

11 Background and Rationale for the Study

Optometry has long been involved in the subject of vision and learning As primary eye care

practitioners optometrists receive referrals from parents teachers psychologists and other

professionals who usually seek the optometristrsquos advice about whether a childrsquos vision problem

may be contributing to or is responsible for poor academic performance

According to Scheiman and Rouse1 learning which is the acquisition of knowledge or skill can

be achieved through study experience or instruction and that between 75 and 90 of what a

child learns is mediated through the visual pathways As vision plays a major role in reading and

the learning process any defects in the visual pathway disrupt the childrsquos learning

ldquoThe three interrelated areas of visual functions are 1

1 Visual pathway integrity including eye health visual acuity and refractive status

2 Visual efficiency including accommodation binocular vision and eye movements

3 Visual information processing including identification and discrimination

spatial awareness and integration with other senses

Several studies2-6 have been conducted to investigate the relationship between vision and dyslexia

on Caucasian populations while very few have been done on African subjects Consequently

most inferences and conclusions on dyslexia are based on research conducted in Caucasian

populations Naidoo7 reported that inferences drawn from studies in Caucasian populations may

not be appropriate for the African population as the lsquoAfrican Eyersquo differs from the Caucasian eye

For example epidemiological studies have reported variations in refractive errors where the

prevalence of myopia is higher among Caucasian persons than in African persons8 9 while

African populations had higher open angle glaucoma prevalence than Caucasian persons10

This study attempts to contribute to the literature by examining the prevalence of a broader

spectrum of vision variables in dyslexic school children in order to investigate a possible

2

relationship between dyslexia and vision in African school children in South Africa A search of

the literature revealed that very little research has been conducted in Africa the only one being

conducted in 1997 by Raju11 Raju11 investigated oculomotor functions in dyslexic school

children in South Africa and reported that 65 of the normal readers exhibited normal

oculomotor control as compared to 11 of the dyslexics Furthermore 54 of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills while 12 of the

controls displayed the same

12 Aim

The aim of the study was to determine the prevalence of vision conditions in a South African

population of African dyslexic children and to study the relationship between vision and dyslexia

13 Objectives

1 To determine the distribution of visual acuity disorders among African dyslexic children

2 To determine the distribution of refractive errors among dyslexic children

3 To determine the distribution of phorias among dyslexic children

4 To determine the distribution of strabismus among dyslexic children

5 To determine the distribution of accommodation disorders among dyslexic children

6 To determine the distribution of vergence disorders among dyslexic children

7 To determine the distribution of ocular pathology among dyslexic population

8 To compare these findings to a similar group of non-dyslexic children

14 Research Questions

1 What is the prevalence of vision conditions among African dyslexic children of age range

10 and 15 years

3

2 What is the relationship between dyslexia and vision

15 Significance of the Study

The study will attempt to

1 Identify high-risk ocular conditions among dyslexic children which might influence the

need for certain examinations or that could affect the childs ability to learn to read

2 Relate findings of this research work to that of previous researchers

3 Add to the knowledge base in the area of vision and dyslexia

16 Goals of Study

1 To obtain quantitative information on the prevalence of visual conditions in dyslexic

children in the African population

2 To obtain information on the relationship between vision disorders and dyslexia

3 To enable improvement of eye care for children with dyslexia

17 Null Hypothesis

1 There will be no statistically significant difference between the mean visual acuity of the

dyslexic group and of the control group

2 There will be no statistically significant difference between the mean refractive errors of the

dyslexic group and of the control group

3 There will be no statistically significant difference between the mean phoria in the dyslexic

group and of the control group

4 There will be no statistically significant difference between the strabismic mean angle of

deviation of the dyslexic group and of the control group

5 There will be no statistically significant difference between the mean accommodation

functions (amplitude facility posture) in the dyslexic group and of the control group

4

6 There will be no statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

1 8 Research (alternative) Hypothesis

1 There will be a statistically significant difference between the mean refractive errors in the

dyslexic group and of the control group

2 There will be a statistically significant relationship between the mean visual acuity of the

dyslexic group and of the control group

3 There will be a statistically significant difference between the mean of phoria of the

dyslexic group and of the control group

4 There will be a statistically significant difference between the strabismus mean angle of

deviation of the dyslexic group and of the control group

5 There will be a statistically significant difference between the mean accommodation

functions of the dyslexic group and of the control group

6 There will be a statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

19 Scope of Study

1 This study is limited to the assessment of visual functions in dyslexic and normal readers

The visual functions investigated include visual acuity refractive error ocular alignment

near point of convergence accommodation functions and vergence reserves The

assessment of eye movement was not part of the study

2 Screening for and diagnosing dyslexia was not part of this study The school

psychologistsrsquo reports were relied on in selecting subjects who qualify to participate in this

study The information was obtained from the learnersrsquo school files 3 The scope of vision parameters investigated is limited only to the prevalence and

relationship between these variables in dyslexic and control groups as outlined in the

hypothesis Correlation analysis of different variables was not part of this study

5

110 Assumptions

In this report dyslexia is used synonymously with ldquodevelopmental dyslexiardquo ldquoreading

disabilityrdquo and reading disorderrdquo and will be used to describe children who find it extremely

difficult to learn to read despite having normal intelligence appropriate educational opportunities

and absence of emotional disorders Children so defined have reading age that is two or more

years behind their chronological age12

111 Study Outline

This thesis is divided into six chapters Chapter one indicates the rationale of the study provides

an overview of dyslexia and details its purpose objectives significance and goals Chapter Two

(the literature review) gives a detailed review of literature in the area of dyslexia learning

disabilities and vision with particular emphasis to vision variables as outlined in the hypothesis

to be tested This is followed by methodology in Chapter Three which details the research

methodology in which a case-control study was done in two schools in Durban The areas of

vision variables evaluated include visual acuity and refraction binocular vision and ocular health

evaluation Chapter Four (Results) presents an analysis of the results obtained from the study

Chapter Five (Discussion) discusses the study findings and their meaning Chapter Six presents

conclusions from the study based on the study results indicating the implications applications

and recommendations for future study and the study limitations

112 An overview of dyslexia

Some children find it difficult to learn to read despite having normal intelligence appropriate

educational opportunities and absence of emotional disorders Their reading age is two or more

years behind their chronological age Such children are referred to as being dyslexics Where

there is an associated deficit in intelligence the condition may be described as generalized

learning disability12

6

The term dyslexia is derived from the Greek words dys meaning hard or difficult and lexia

from the word lexicos which means pertaining to words so dyslexia means difficulty with

words-either seen heard spoken or felt as in writing13

The World Federation of Neurologists13 and the International Classification of Diseases-1014

defined dyslexia as ldquoa disorder manifested by difficulty in learning to read despite conventional

instruction adequate intelligence and socio-cultural opportunity It is dependent upon

fundamental cognitive disabilities which are frequently constitutional in originrdquo

Dyslexia is a complex disability which affects different aspects of reading The acquisition of

reading related skills are coordinated by visual motor cognitive and language areas of the brain

Consequently dyslexia can result from deviation of normal anatomy and function of cognitive

and language areas in the brain 15

Dyslexia refers to a cluster of symptoms resulting in people having difficulties in specific

language skills particularly reading15 The mystery in dyslexia as a syndrome lies in the fact that

dyslexics have ldquodifficulty to learn to read despite having average or above average intelligence

adequate auditory and visual acuity absence of frank brain damage no primary emotional

problems and have adequate educational instructionsrdquo 16

Dyslexia is a developmental disorder that shows in different ways at different developmental

stages As documented by Krupska and Klein 13 dyslexia is a learning disability characterised by

problems in expressive or receptive oral or written language that manifest initially as difficulty in

learning letters and letter sound association which is followed by difficulty in learning letters and

reading words accurately which consequently results in impaired reading rate and written

expression skills (handwriting spelling and compositional fluency) Some dyslexics find it

difficult to express themselves clearly or to fully understand what others mean when they speak

The effects of dyslexia reach well beyond the classroom as it can affect the persons self-image

It makes them feel dumb13 17 Dyslexia describes a different kind of mind often gifted and

productive that learns differently An unexpected gap exists between learning aptitude and

achievement in school 18

Many dyslexics are creative and have unusual talent in areas such as art athletics architecture

7

graphics electronics mechanics drama music or engineering 18 Some dyslexic learn best with

a multi-sensory delivery of language content which employ all pathways of learning at the same

time seeing hearing touching writing and speaking18 It is primarily not a problem of

defective vision although underlying vision deficit has been known to exaggerate the dyslexics

reading problems 18

According to Critchley19 ldquothe child otherwise normal as regards to intellectual status and

emotional stability finds an isolated difficulty in mastering the significance which lies behind

printed or written symbols and of necessity finds it extremely hard to align the verbal

components of speech (as emitted by the mouth and as received by the ear) with the empirical

characters which are set out on paper in one form or anotherrdquo Dyslexics are unable to acquire

written language skills through ordinary learning and teaching methods and often fail to progress

in education19

A dyslexic individual displays a characteristic pattern of decoding (recognising) and encoding

(spelling) difficulties with written words20 Interestingly dyslexics may not have overt

difficulties with spoken language yet do have marked difficulties with written languages21 The

difference between written and spoken language is that written language is seen while spoken

language is heard This fact may explain why vision has always been implicated on the topic of

dyslexia 21

Dyslexics experience difficulties with visual-verbal matching necessary to establish word

recognition ability Even when a clear single visual image is present the dyslexic may be unable

to decode the printed word in some cases This may be related to the fact that the difficulty that

dyslexics have is at a much more fundamental stage in the reading process so the role of vision

function is quite different in dyslexia than in other types of reading difficulty22 (which may be

peripheral such as hyperopia leading to difficulty in reading) The two main situations in which

the term dyslexia now commonly applies are when the reader has difficulty decoding words (that

is single word identification) and the second is encoding words (spelling) The second type is a

frequent presentation in optometric practice that is when the reader makes a significant number of

letter reversals errors (example b-d) letter transposition in word when reading or writing

(example sign for sing) or has left-right confusion 23

8

113 The Reading Process

Language development entails four fundamental and interactive abilities listening speaking

reading and writing while learning to read involves a complex system of skills relevant to visual

(the appearance of a word) orthographic (visual word form) phonological and semantic

(meaning of word and phrases) processing and a variety of behaviors such as letter naming letter

perception word recognition and comprehension each of which uses a different part of the brain

Reading is an interpretation of graphic symbols24-26

For some children the ability to break a word into its smaller parts a task crucial in reading is

extremely difficult Dyslexia is a localized problem one involving the sounds and not the

meaning of spoken language Speaking is natural while reading is different it must be learned

To be able to transform the printed letters on the page to words that have meaning the letters

must be connected to something that already has inherent meaning- the sounds of spoken

language In order to learn to read children must learn how to link the printed letters on the page

to the sounds of spoken language and understand that words are made up of sounds and that

letters represent these sounds or phonemes24- 26 In other words learning to read requires that the

child be able to break the word into the individual components of language represented by letters

and to be able to tell the difference between the individual phonemes that make up a word

Learning to read requires that the central principle behind the alphabet is understood that is

words are made of phonemes and in print phonemes are represented by letters Phonemes are

the shortest units of sound that can be uttered in a given language and that can be recognized as

being distinct from other sounds in the language For example the word cat has three phonemes

ca ah and ta Blended together the sounds form the word rsquocatrsquo24-26 When one speaks the

sounds are blended ttogether and are said one cat 24-26 To learn to read a child must learn that

there are three separate sounds This is difficult for children with dyslexia The inability to break

word into its parts is the main reason why children with dyslexia have trouble learning to

read24- 26

Furthermore dyslexic children find it difficult to bring the print to language (such as when asked

to read what they just wrote) For example a child can copy the letters w-a-srdquo correctly but

9

when asked what was written a child with dyslexia may reply saw The problem in this case

may not be related to vision but rather one of perceptual skills of what the child does with a word

on a page The brain mechanism of going from print to language is phonologically based 24-26

Reading acquisition builds on the childrsquos spoken language which is already well developed

before the start of formal schooling Once a written word is decoded phonologically its meaning

will become accessible via the existing phonology-to-semantics link in the oral language system

Thus the childrsquos awareness of the phonological structure of sound of speech plays a major role in

the development of reading ability 24-25

114 Eye Movement in Reading

For a person to be able to read a letter or word the eye has to first identify the written material

Normally the eyes scan across the line of words on the page stopping to fixate at various points

then making short jumps from one position to the next (some saccade eye movements are

involved) Typically there are around five fixation pauses in a line Sometimes the eye will

regress and go back to a previously passed over word element A reader views the text mainly

with the controlling or reference eye whilst the other eye diverges and converges within fine

elements27 A sophisticated mechanism27 (a connection of the visual receptors of the retina with

the visual nervous pathway to the brain) 28 ensures control of this drift and prevents diplopia from

being consciously observed These drifts and subsequent recovery can be clearly seen by a

number of techniques including infrared observation of the eyes27

The minimum time a person can spend on fixation and move to the next is around 250

milliseconds27 For skilled readers the average saccade length is 7-9 letter spaces and the

duration of saccades is of the order of 25-50 milliseconds29 while the average fixation pauses last

between 200-250 milliseconds27 29 Thus our eyes are still for some 90 of the time while

reading and are subject to large changes in acceleration and deceleration Even a good reader

will regress and go back 10 to 20 of the time27 29

The textual image seen by the eyes is imaged onto both maculae and transmitted to both cerebral

10

cortex where it is first received in the primary visual cortex (Brodmanrsquos area 17) with some

initial processing of colour and tone occur Progressively higher levels of processing occur in

adjacent areas 18 and 19 with the highest level of visual language processing taking place in the

angular gyrus (area 39) The angular gyrus is central to reading since it is here that hearing

speech and vision meets and the written word is perceived in its auditory form The auditory

form of the word is then processed for comprehension in Wernickersquos area as if the word had been

heard25 27

115 Characteristic SignsSymptoms of Children with Dyslexia

Dyslexia is heterogeneous in nature and there is no single pattern of difficulty that affects all

people Dyslexic children frequently show a combination of characteristics (Table 11)

Table 11 Characteristic SignsSymptoms of Dyslexia 16 30-31

Reading Visionminus Holds book too close

minus Word reversals

minus Skips complete words

minus Re-reads lines

minus Points to words

minus Word substitution

minus Sees double

minus Poor comprehension in oral reading

minus Might see text appearing to jump around on a page

minus Unable to tell difference between letters that look similar in shape such as o and e

minus Unable to tell difference between letters with similar shape but different

orientation such as b and p and q

minus Letters might look jumped up and out of order

minus Letters of some words might appear backwards eg bird looking like drib

11

Spelling

minus Omission of beginning or ending letters

minus Can spell better orally than written

minus Letter reversals

minus Wrong number of letters in words

Difficulty with Reading

minus Difficulty learning to read

minus Difficulty identifying or generating rhyming words or counting syllables in

words (Phonological awareness)

- Difficulty with manipulating sounds in words

minus Difficulty distinguishing different sounds in words (Auditory discrimination)

AuditoryVerbalminus Faulty pronunciation

minus Complains of ear problems

minus Unnatural pitch of voice

minus Difficulty acquiring vocabulary

minus Difficulty following directions

minus Confusion with beforeafter rightleft

minus Difficulty with word retrieval

minus Difficulty understanding concepts and relationships of words and sentences

116 Implications of Dyslexia for Learning

Dyslexia affects people in different ways and depends on the severity of the condition as well as

the effectiveness of instruction or remediation The core difficulty is with word recognition and

reading fluency spelling and writing Some dyslexics manage to learn early reading and spelling

tasks especially with excellent instruction but later experience their most debilitating problems

when more complex language skills are required such as grammar understanding textbook

material and writing essays The dyslexic syndrome therefore affects reading and learning in the

12

following ways263034

1161 Reading and Comprehension

One major difficulty dyslexics have is the time it takes to read and understand a text This makes

reading an extremely difficult task and they may report that they get tired and may get headaches

after reading for a short while30 There is also difficulty in assimilating what they have read even

when they know the words Multiple-choice questions are usually difficult for them as they find

it difficult to recognise words out of context263034

1162 Handwriting

Dyslexics have problems with handwriting since for some forming letters takes concentration

that their ideas get lost and their written expression suffers30

1163 Proof-reading

Dyslexics with visual processing difficulties usually have difficulty in identifying errors in

their own writings Reading difficulties make it uneasy for them to see their own errors in

expression and sentence structure grammar26 30

117 Prevalence of Dyslexia

Estimates of the prevalence of dyslexia in Caucasian populations vary from 5 to 20 in the

general population16 Pavlidis 31 reported that about 1 to 3 of the total population of children

suffers from dyslexia and that 20 to 30 of those cases classified as ldquogeneral reading failures

are probably dyslexics Evans32 reported a prevalence of 4 - 5 in children and stated probably

more boys than girls are dyslexic Park33 documented that up to 15 of the school-age

population has some degree of dyslexia with a ratio of four boys to one girl The large variability

in the prevalence reported by different authorities may be attributed to the differences in the

diagnostic criteria and the cut-off point applied to the psychometric tests14 Dyslexia is the most

common and the most researched type of learning disability34

13

A literature search revealed one study in Egypt that reported the prevalence of specific reading

difficulty in 2nd and 3rd grades in elementary school population to be 1 and the male to female

ratio to be 27 to 1 The authors concluded that the prevalence was low compared to that reported

in western countries and that the difference may be due to the way the Arab language is written

and read 35

Contrary to the above reports that males tend to be more dyslexic than females Guerin Griffin

Gottfried and Christenson 36 studied dyslexia subtypes and severity levels with respect to gender

differences The authors36 concluded that

when objective criteria such as the discrepancy between achievement (in

reading andor spelling) and intelligence or direct assessment of coding

skills are used as criterion and non-referred or random samples are

assessed the ratio of males to females identified as dyslexics approaches

11 When identification depends on referrals from parents and or

teachers more males than females may be detected because of the nature

and intensity of boysrsquo behavioural reactions to the disability

According to Shaywitz and Shaywitz34 good evidence based on sample surveys of randomly

selected population of children now indicates that dyslexia affects boys and girls comparably

118 Aetiology of dyslexia

The literature contains diverse theories (not less than four major theories) on the aetiology of

dyslexia probably due to the complex nature of the subject but several authors13-15 26 37-42 have

maintained that it is a neurological disorder with a genetic origin Despite decades of

multidisciplinary research neuropsychology brain anatomy and neuro-imaging 37-42 the specific

causes of dyslexia are still unknown The leading assumption however is that dyslexia

fundamentally stems from subtle disturbances in the brain and other possible explanations of the

cause of dyslexia such as language processing difficulties are based on the neurological etiology

14

The aetiological considerations in dyslexia are

1181 Neurological Factors

As documented by Habib 37 the concept of the neurological basis of dyslexia was first mentioned

independently by a Scottish ophthalmologist Hinshelwood in 1895 and a British physician

Morgan in 1896 Both found similarities of certain symptoms of dyslexia in some children

Dyslexia was then described as ldquovisual word blindnessrdquo The earlier understandings for the

neurological basis of dyslexia came from neuro-pathological studies of brains from dyslexic

persons According to Habib37 it was first reported by the French neurologist Dejerine in

1891 that damage to the left inferior parieto-occipital region (in adults) resulted in variable

degree of impairments in reading and writing suggesting that the left angular gyrus may play a

role in processing the ldquooptic images of letterrdquo It was subsequently thought that impaired reading

and writing in the young dyslexic patients could be due to abnormal development of the same

parietal region which was damaged in adult alexic patients 37

Another perspective 13 37 is that the dense distributions of ectopias (the presence of neural

elements in places where they are not supposed to be found) all over the cerebral cortex

(particularly in the perisylvian language areas) resulted in alteration of brain organisation One

such alteration in dyslexia is the lack of asymmetry in a language-related cortical region called

the planum temporale (an auditory area that lies on the superior surface of the temporal lobe) In

normal subjects the platinum temporale is usually larger in the left hemisphere However the

dyslexics showed symmetry of the planum temporale 13 37 Furthermore these ectopias may

reduce cortical connectivity as suggested by recent positron emission tomography and magnetic

resonance imaging studies 37

Another speculation was that the development of ectopias could be due to foetal hormonal

(possibly testosterone) imbalances during late pregnancy which could also account for the

possible male predominance in dyslexia 15 30 37 38

The maturation lag which is believed to be due to the slower development of the nerve fibres of

the corpus collosum (bundle of cells connecting the left and right hemisphere of the brain) has

also been theorized to be a cause of dyslexia A delay in the growth of the fibres linking the two

15

hemispheres would result in affected children being physiologically incapable of tasks which

require hemispheric organisation This leads to the inadequate development of the language

functions 12 37 However recent neuro-imaging studies 39-42 have shown that impaired reading of

alphabetic scripts (such as in the English language) is associated with dysfunction of the left

hemisphere posterior reading systems primarily in the left temporo-parietal brain regions

1182 Genetic Factors

Genetic considerations in dyslexia are derived from the fact that dyslexia runs most often in

families However it is speculated that different forms of dyslexia may occur within the same

family which also may means that it seems likely that the inheritance is indirect 15 31 However

genetic markers have been identified in chromosome 18 38 and chromosomes 6 and 15 15 In

fact chromosomes 2 (DYX3) 6 (DYX2) and 15 (DYX1) in dyslexics have been reported to be

inherited in an autosomal dominant mode of transmission 15 Furthermore Ramus 43 stated that

ldquogenetically driven focal cortical abnormalities such as ectopias and microgyri in specific areas

of left perisylvian cortex involved in phonological representations and processing are the

primary cause of dyslexia which is consistent with anatomical studies of dyslexic brains showing

loci of cortical abnormalities functional brain imaging studies showing that the very same areas

are involved in phonological processing and show abnormal activation in dyslexicshelliprdquo

1183 Peri-natal Factors

Peri-natal factors may manifest as problems during pregnancy or complications after delivery

More importantly it can also happen that the mothers immune system reacts to the foetus and

attacks it as if it was an infection It has also been theorized (based on partial evidence) that

dyslexia may occur more often in families who suffered from various autoimmune diseases (such

as rheumatoid arthritis Gravesrsquo diseases multiple sclerosis systemic lupus Sjogrenrsquos

syndrome)3137

1184 Substance Use during Pregnancy

16

Drugs such as cocaine (especially in its smokable form known as crack) seem to affect the

normal development of brain receptors of the foetus It has been documented that mothers who

abuse substance during pregnancy are prone to having smaller babies and such infants are

predisposed to a variety of problems including learning disorders Thirdly alcohol use during

pregnancy may influence the childrsquos development and lead to problems with learning attention

memory or problem-solving There is also a risk of foetal alcohol syndrome (characterised by low

birth weight intellectual impairment hyperactivity and certain physical defects) 30

1185 Environmental Factors

Environmental factors are risk factors for the development of reading disabilities 44 Reading

disabilities which are due to environmental influences have been referred to as non-specific

reading disabilities 20 or pseudo- dyslexia as the reading disorder is secondary to extraneous

factors and genetic factors play an important role 45 ldquoEnvironment factors play a significant role

in the level to which neurological impairments manifest themselves Encouraging nurturing and

stimulating surroundings can reduce the impact of risk factors on learning performance and life

skillsrdquo 44 Environmental circumstances which may profoundly influence the dyslexic child

include parental attitude the role of multilingualism in the home attention problems the

drawback of frequent changes of school the personality of the teacher and the childs emotional

reaction to his difficulties 152030

It seems that the influence of environmental factors may best be described as a potential risk

factor than a causative factor in dyslexia as there are diverse views on the relationship between

environmental influences and dyslexia According to Grigerenko46 socioeconomic level

educational opportunity and home literacy level are major environmental risks for the

manifestations of developmental dyslexia hellip the ldquomagnitude of the effects was not strong enough

to view them as powerful causal factorsrdquo

1186 Nutrition

Nutrition plays an important role in bridging the gap between genetic constitution and a childs

potential for optimal development Nutrition is a basic requirement for the maintenance of

17

growth and overall development while malnutrition and under-nutrition are associated with

disorders in the normal development of intelligence perceptual maturation and academic

achievements 47 Studies conducted by Grant Howard Davies and Chasty 48 showed that there

is an association between dyslexia and low concentration of zinc Zinc is necessary for normal

brain development and function and essential for neurotransmission44

119 Diagnosis of Dyslexia

The first stage in the diagnosis of dyslexia is with awareness by parents or educators that a

problem in reading exists The initial point of assessment starts with the medical practitioner

(possibly a paediatrician) who conducts a complete medical examination and obtains a

comprehensive health history A correct diagnosis of dyslexia starts with a very well taken case

history especially good awareness of the childrsquos developmental history This may give

indications of any medical condition that may be contributing to reading difficulties If indicated

the child may be referred for a neurological examination If dyslexia is suspected the physician

may then refer the child for further evaluation and treatment by a specialist in psycho-educational

assessments In most cases the diagnosis of dyslexia is usually performed by the psychologist

The Psychologists conduct psychometric assessments which include assessment of intelligence

quotient (IQ) various aspects of reading performance spelling mathematics sequential memory

and other aspects of cognitive and emotional development49 50 The major purpose of the

diagnostic process is to isolate the specific difficulties associated with dyslexia and to suggest

appropriate educational intervention 49 50

There are three methods for diagnosing dyslexia

1191 Exclusionary Method

In the exclusionary method the diagnosis of dyslexia is made when all factors involved in a

nonspecific reading disability (reading problems resulting from factors such as intellectual

disability dysfunction of hearing or vision inadequate learning experience socio-cultural

deprivation primary emotional problems poor motivation) are excluded 2051 The problem with

18

the exclusionary method of diagnosing dyslexia is that dyslexia cannot be diagnosed until after

the child has been failing in school for almost two years By this time constant failure may have

produced a negative attitude towards school and psychological problems 31

It provides a very

limited description of the features of the disorder Dyslexia is often defined in terms of the

discrepancy between reading achievement and chronological age or grade level failing to

consider the possible overlap between some of the intelligence quotient tests and reading tests 52

According to Snowling 53 IQ is not strongly related to reading and so the use of IQ in defining

dyslexia is inappropriate stating that many children with low IQ can read perfectly well and that

the discrepancy definition (based on the difference between reading achievement and

chronological age) of dyslexia cannot be used to identify younger children who are too young to

show a discrepancy

1192 Indirect Method

The second method of diagnosing dyslexia is the indirect approach A diagnosis is sought by

attempting to associate neurological soft signs such as finger ldquoagnosiardquo with reading failure The

drawback with this method is that many dyslexics do not manifest neurological soft signs 20

Another type of the indirect method involves the analysis of cognitive tests An example is the

Wechsler Intelligence Scale for Children (Revised) (WISC-R) WISC-R which compares ldquoverbalrdquo

versus ldquoperformancerdquo 20

1193 Direct Method

The third method which seems most reliable is the direct method which involves the use of

characteristic decoding and spelling patterns to determine the specific dyslexia It has the

additional advantage of addressing the fact that dyslexia is heterogeneous 20

19

120 Functional Imaging Techniques used in the Diagnosis and Study of

Dyslexia

There are functional imaging techniques for diagnosing and studying brain activity in dyslexia

These techniques include positron emission tomography (PET) Magnetic resonance imaging

(MRI) and computed tomography (CT) The two major techniques include

1201 Positron Emission Tomography (PET)

PET is a non-invasive imaging technique that uses radioactively labeled compounds to

quantitatively measure metabolic biochemical and functional activity in living tissue It

assesses changes in the function circulation and metabolism of body organs 54 55 The PET is

a sophisticated nuclear medicine technique that looks directly at cerebral (brain) blood flow

and indirectly at brain activity (or metabolism) The most commonly used labels are

flourophores which emit light when stimulated with light of the appropriate wavelength and

radio nuclides which emit gamma rays or beta particles when they decay56 The dose of

radionuclide injected is minute and does not pose any significant hazard Various isotopes are

used depending upon the brain region and function studied54

Unlike MRI or CT imaging which primarily provide images of organ anatomy PET measures

chemical changes that occur before visible signs of disease are present on CT and MRI images 54 While PET is most commonly used in the fields of neurology oncology and cardiology

applications in other fields are currently being studied 57

1202 Functional Magnetic Resonance Imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a noninvasive usually painless procedure that

uses a powerful magnetic field radio waves and a computer to produce detailed pictures of

organs soft tissues bone and virtually all other internal body structures The images can then be

examined on a computer monitor or printed The fMRI does not use ionizing radiation57 58

Functional magnetic resonance imaging (fMRI) is a relatively new procedure that uses MR

imaging to provide a picture of the brains active rather than its static structure The fMRI

20

machines measure the level of oxygen in the blood through a technique called blood level-

dependent (BOLD) contrast The two different states of hemoglobin-oxygen-rich oxyhemoglobin

and oxygen-poor deoxyhemoglobin differ in their magnetic properties The huge fMRI magnets

are sensitive to changes in the concentration of deoxyhemoglobin As neural activity increases

blood flow to the vasculature of the brain presumably increases altering this concentration The

more active a brain area is the more blood flows to it Thus fMRI can provide a moment-by-

moment movie of brain activity

Studies using fMRI are beginning to shed light on dyslexia Functional magnetic resonance

imaging (fMRI) is similar to MRI as both procedures use a similar scanner The difference is that

the fMRI uses somewhat more sophisticated hardware and software that allow it to capture brain

changes (mainly blood flow) as a person performs a specific cognitive task The functional

magnetic resonance imaging (fMRI) scanner measures where blood flows through the brain

during certain tasks26

Earlier studies of dyslexia were based on histo-pathological studies According to Habib37 the

fMRI has an added advantage that cortical anatomy of the dyslexic brain can be demonstrated

thus answering the criticism addressed to results from pathological findings

120 Classifications of Dyslexia

Dyslexia can be classified into two broad categories according to its presumed aetiology The

following methods of classification have been documented by Helveston59

1 Primary dyslexia (specific developmental dyslexia)

2 Secondary dyslexia

1211 Primary Dyslexia

This is considered to be caused by a specific central nervous system defect This neural defect is

thought to be located in the angular gyrus of the dominant hemisphere It is often hereditary and

21

may affect more boys than girls In primary dyslexia the decoding process involved in language

system is affected and reading becomes extremely difficult for the child However intellectual

tasks that do not involve reading text are unaffected 59

1212 Secondary Dyslexia

There are two types of secondary dyslexia59 Endogenous and Exogenous

a Endogenous Dyslexia this results from pathological changes in the central nervous

system secondary to trauma or disease such as childhood meningitis hydrocephalus

with brain damage and porencephalus59 Other medical causes include prematurity

congenital hydrocephalus encephalitis traumatic brain injury and lead or

methylmercury poisoning50

b Exogenous Dyslexia this results from intellectual deprivation usually limited to lack of

adequate educational opportunity or motivation59 or due to low intelligence socio-

cultural deprivation primary emotional problems sensory impairment (visual or

auditory) poor motivation or attention problems20 Reading disability due to these

mentioned factors have been referred to as non-specific reading disability20

1 22 Sub-types of Dyslexia

When children first start to read they learn to recognise simple words by their shapes (sight

analysis) building their sight vocabulary This is followed by learning how to analyse

complicated words by breaking them down into their sound components (phonetic analysis)

The dynamics of the two processes (Sight-analysis and Phonetic analysis) are important in

explaining how words are decoded based on the neurologic-behavioural model documented by

Christenson et al 20 and Evans 21 Dyslexia can be divided into the following subtypes 20 21

22

1221 Dysphonetic (auditoryphonetic)

The individual has a minimal dysfunction involving the Wernickes area Readers have an

extremely difficult time developing phonic word analysis or decoding skills Their typical

reading errors are wild guesses (lsquolikersquo for lsquolittlersquo) errors picking words with similar shapes

(horse for house ) and semantic substitutions (home for house) Their typical spelling errors

are phonetically inaccurate (reffor rough) This type of reader persists in using whole-word

approach and will guess at unfamiliar words rather than using their word analysis skills While

they do not often continue to reverse letters and words after the age of eight they often make

spelling reversals (For example interver for inventor and wirters for writers) Typically

they often look at the first and last letter and the length of the word and then guess (monkey for

money or stop for step)20 21

1222 Dyseidetic (visual-spatial)

In dyseidetic type of dyslexia dysfunction occurs in the angular gyrus The individual have high

ability in phonic word analysis They often have a difficult time learning the letters of the

alphabet They usually have a very low sight vocabulary poor visual memory poor perceptual

skills and letter and word reversals (dig for big or saw for was) Their reversals often last

past the age of eight and include both reading and writing reversals This type of child is often a

very slow reader who often will try to sound out familiar as well as unfamiliar words They spell

the word as it sounds By first identifying the learning style of these children probably during the

case history the optometrist may know the childs strengths and weaknesses Vision therapy can

then be designed to fit each childs needs Enhancing the childrsquos visual perceptual and eye

tracking skills plus having the schools teach these children according to their strengths enable

them to achieve their full potential in school 20 21

122 3 Dysnemkinesia (motor)

Dysnemkinesia is a less serious form of dyslexia and it involves minimal dysfunction of the area

of the motor cortex involved in letter formation Dysnemkinesics can be distinguished by their

23

frequent letter reversals such as d for b as in doy for boy Additional dyslexic types occur

when combinations of the three basic types arise such as dysphoneidesia dysnemkineidesia

dysnemkinphonsia and dysnemkinphoneidesia20 21

1224 Dysphonetic-Dyseidetic

Dysphonetic- dyseidetic reader has a weakness in both whole-word and phonic analysis Their

reading errors include wild guesses (fish for father) and word reversals (no for on) Their

spelling is usually phonetically inaccurate and often bizarre (rit for faster) 2021

123 Theories of Dyslexia

Assisted by the increasing body of knowledge obtained by neurophysiological and imaging

studies14 several theories have been proposed with the aim of characterizing the fundamental

processes underlying dyslexia14 These theories can be classified under four major frameworks

phonological visual cerebellar and auditory

1231 PhonologicalndashDeficit Theory

According to the phonological theory affected individuals have difficulties in perceiving and

segmenting phenomes leading to difficulties in establishing a connection between phonemes and

graphemes 14 The brain recognizes language in a hierarchical order The upper levels deal with

semantics (the meaning of words) syntax (grammatical structure) and discourse (connected

sentences) The lower levels of hierarchy deal with breaking sounds into separate small units

called phonemes (sound units) Thus before words can be comprehended at higher levels in the

hierarchy it has to be broken down into phonologic constituents that the alphabetic characters

represents15 26 An adequate reading development stems from some considerable level of spoken

language already acquired by a child in the early years of life Consequently failure to develop

the association between letters (grapheme) and sound (phoneme) has been considered to be a

major cause of reading and spelling impairment in most cases of developmental dyslexia60-62

24

Reading requires some skills of phonological processing to convert a written image into the

sounds of spoken language1 According to the phonologic-deficit hypothesis people with

dyslexia have difficulty developing an awareness that words both written and spoken can be

broken down into smaller units of sound and that in fact the letters constituting the printed word

represent the sounds heard in the spoken word2

According to Habib37 neuropsychological studies have provided considerable evidence

indicating that the main mechanism leading to reading difficulties is phonological in nature and

that it has been repeatedly demonstrated that the core deficit responsible for impaired learning to

read is phonological in nature and has to do with oral language rather than visual perception At

the brain level this cognitive deficit would arise from a congenital dysfunction of certain cortical

areas involved in phonology and reading60 At the neurological level it is usually assumed that

the origin of the disorder is a congenital dysfunction of the left-hemisphere perisylvian brain

areas underlying phonological representations or connecting between phonological and

orthographic representations60

Brain imaging studies in dyslexics in response to a phonological task indicate under-activation of

posterior brain regions (Wernickesrsquos area angular gyrus and striate cortex) and relative over-

activation in anterior regions (inferior frontal gyrus) These brain activation patterns provide

evidence of an imperfectly functioning brain system for segmenting words into their phonologic

constituents39 42 Since most dyslexics show deficits in phoneme processing the phonological

deficit is the most significant consistent marker of dyslexia and remains the predominant theory15343739 hence the focus on phonology in the majority of remedial programmes 63

Based on the phonological deficit theory the other symptoms of dyslexia are considered as

simple co-morbid markers that do not have a causal relationship with reading disability62

However proponents of the phonological theory typically contend that these disorders are not

part of the core features of dyslexia62 A major setback with the phonological theory is that it

does not account for symptoms such as impairment in visual perception and problems with motor

coordination that are found in dyslexics 15 and does not account for symptoms of dyslexia that are

unrelated to phonetic decoding difficulties such as problems with short term memory and visual

25

processing issues64

The two basic processes involved in reading are decoding and comprehension In dyslexia a

deficit at the level of the phonologic units impairs the ability to segment the written word into its

underlying phonologic elements Consequently the reader experiences difficulty initially in

decoding the word and then in identifying it The phonologic deficit is independent of other non-

phonologic abilities especially the higher-order cognitive and linguistic functions involved in

comprehension such as general intelligence and reasoning vocabulary and syntax are generally

intact A deficit in phonologic analysis and an ironical intact higher-order cognitive ability offers

an explanation for the paradox of otherwise intelligent people who experience great difficulty in

reading 26

According to the phonological model a deficit in a lower-order linguistic (phonologic) function

prevents access to higher-order processes and to the ability to draw meaning from text The

problem is that the person cannot use his or her higher- order linguistic skills to access the

meaning until the printed word has first been decoded and identified Suppose for example that

a man knows the precise meaning of the spoken word apparition however until he can decode

and identify the printed word on the page he will not be able to use his knowledge of the

meaningof the word and it will appear that he does not know the words meaning 26

1232 Cerebellar Theory

The normal pattern of cerebellar asymmetry is anomalous in dyslexia 13 The cerebellar deficit

theory suggests that the automatisation of cognitive processes and motor control in the

cerebellum are disturbed in individuals with dyslexia14 15 62 Evidence for this theory comes

from poor dyslexic performance in coordination balance and time estimation The cerebellar

theory is related to cerebral dominance (greater proficiency of each cerebral hemisphere in

acquisition and performance of certain specific functions) and lateralization (one-sided location

of brain function)13

Physiologically the brain operates cross-laterally thus the left part of the brain controls the right

side of the body and vice versa The human brain is normally asymmetrical due to the

26

specialized functions in the left and right brain However the dyslexic brain appears to be more

symmetrical than the normal brain The consequence of this is that both sides of the brain may

be competing to handle language and therefore less efficient in handling processing language

The left hemisphere is especially important for the production of language The right hemisphere

also has some language capabilities but has no ability to analyse speech sound Secondly the

cerebellum plays a role in motor control and therefore in speech articulation It is postulated that

retarded or dysfunctional articulation would lead to deficient phonological representations

Furthermore the cerebellum plays a role in the automatization of tasks such as driving typing

and reading A weak capacity to automatize would affect among other things the learning of

grapheme-phoneme correspondences 13-15 62

1233 Visual Deficit Theory

For many years a major view was that reading difficulties are caused by dysfunction in the

magnocellular subsystem 65-67 The visual theory does not exclude the phonological deficit but

emphasizes a visual contribution to reading problems in some dyslexics The biological basis of

the proposed visual theory is derived from the division of the visual system into two sub-systems

which carry information to the visual cortex the transient and the sustained systems62

Anatomically the magnocellular pathway receives input from both rods and cones across the

retina and extends from ganglion cells in the retina to the two ventral layers of the lateral

geniculate nucleus (LGN) The large cell bodies in these two layers give the pathway its name It

projects via the visual cortex and the dorsal or ldquowhererdquo stream to the parietal cortex The

parvocellular (or sustained) system on the other receives input from cones and therefore mainly

the central retina and runs from retinal ganglion cells to the smaller cell bodies of the four dorsal

layers of the LGN It projects via the visual cortex and the ventral or ldquowhatrdquo stream to the

infero-temporal cortex and mediates colour vision and detection of fine spatial details61 The

magnocellular system is the first to respond to a stimulus and relies on further detail from the

parvocellular system (Table 12)

27

Table 12 Features of the Magnocellular and Pavocellular Systems2168

Magnocellular(transient) System Pavocellular (sustained) SystemLarge cells Small cells

Dim light Bright light

High contrast sensitivity Low contrast sensitivity

Low spatial frequency High spatial frequency

Peripheral Central

Right brain Left brain

Midbrain Cortical

Non-cognitive Cognitive

Emotional Logical

Hyperopia Myopia

Gross depth perception Stereopsis

Achromotopsia Colour vision

Fast transmission Low transmission

Sensitive to large stimuli Sensitive to small stimuli

The transient system is vital to the timing of visual stimuli and plays an important part in

detecting the movement of objects (motion perception) and ultimately their signals are used to

control eye and limb movements made in relation to visual targets1521 Impaired function of

magnocellular pathway will lead to destabilization of binocular fixation which results in visual

confusion and letters then appear to move around

Dyslexicsrsquo eyes are unsteady when they are attempting to view small letters hence their vision is

unstable they tend to make visual reading errors15 There is a tendency to superimpose letters on

top of each other and mis-sequence letters in a word1621 Evidence for magnocellular

dysfunction comes from anatomical studies of dyslexic individuals showing abnormalities of the

lateral geniculate nucleus1562 psychophysical and neuroimaging studies 62 64 showing decreased

sensitivities in the magnocellular range (low spatial frequencies and high temporal frequencies)

in dyslexics

28

A deficit in the magnocellular system slows down the visual processing system resulting in an

unstable binocular control inaccurate judgements of visual direction a tendency to superimpose

letters and mis-sequence of letters in a word16 21 Some studies65-67 have shown evidence in

support of the magnocellular deficit theory of dyslexia

1234 Auditory Processing Theory

The auditory processing theory specifies that the deficit lies in the perception of short sounds and

fast transitions (called the ldquorapidrdquo or ldquotemporalrdquo auditory processing deficit) Such a

characterization of the auditory dysfunction is consistent with the magnocellular theory since

magnocellular cells are particularly sensitive to high temporal frequencies60 62

Proponents of the auditory processing theory hypothesized that impaired perception of brief

sounds and transitions would be particularly detrimental to speech perception hence would

undermine the development of the childrsquos phonological representations Contrary to this

hypothesis Rasmus60 reported that recent studies have now adequately confirmed that there is

no reliable relationship between performance on rapid auditory processing tasks and speech

categorization and discrimination neither is there a reliable relationship between any auditory

measure (speech or non-speech) and more general measures of phonological skill or reading

ability even when assessed longitudinally A possibility is that the most auditorily impaired

dyslexics also have severely impaired phonology and reading although the reverse is not

necessarily true 60

Despite the fact that evidence exist in support of the theories and that some of the deficits found

in affected individuals are correlated with reading and spelling they may not be causally

associated with dyslexia However results from genetic research may have the potential to help

delineate which cognitive and neurophysiological processes are causally related14

124 Cultural Differences in Dyslexia

The recognition of dyslexia as a neuro-developmental abnormality has been influenced by the

belief that it is not a specific diagnostic entity because it has variable and culture-specific

29

manifestations 69-71 Recent neuro-imaging studies39-42 have demonstrated that impaired reading

of alphabetic scripts is associated with dysfunction of left temporo-parietal brain regions

However it has been reported by Siok et al in two separate studies69 70 that in contrast to the

assumption that dyslexia in different languages has a universal biological origin their study using

functional magnetic resonance imaging with reading-impaired Chinese children and associated

controls showed that functional disruption of the left middle frontal gyrus is associated with

impaired reading of the Chinese language (a logographic rather than alphabetic writing system)

The neural connections involved in reading and reading disorders may vary in different languages

because of differences in how a writing system links print to spoken language69 70 In Chinese

for example graphic forms (characters) are mapped to syllables which differ markedly from an

alphabetic system (such as English) in which graphic units (letters) are mapped to phonemes

These differences can lead to differences in how reading is supported in the brain69 70 Readers

of Chinese show relatively more engagement of visuo-spatial areas and left middle frontal

regions for verbal working memory presumably for recognizing complex square-shaped

characters whose pronunciations must be memorized instead of being learned by using letter-to-

sound conversion rules69 70

In two separate studies Siok Perfetti and Tan69 and Siok Zhen Charles Perfetti and Tan 70

studied language-related activation of cortical region of Chinese dyslexic school children using

the functional magnetic resonance imaging and found a reduced activation in the same left

middle frontal gyrus region in Chinese dyslexics Chinese dyslexics by contrast did not show

functional or structural differences from normal subjects in the more posterior brain systems that

have been shown to be abnormal in alphabetic-language dyslexics69-70 The authors concluded

that the results of the studies suggested that the structural and functional basis for dyslexia varies

between alphabetic and non-alphabetic languages and that the findings provided an insight into

the fundamental patho-physiology of dyslexia by suggesting that rather than having a universal

origin the biological abnormality of impaired reading is dependent on culture

The study by Paulesu Demonet Fazio and McCrory71 gave another perspective Using the

positron emission tomography to study brain activity of dyslexics from three countries (Italian

French and English) the authors found the same reduced activity in a region of the left

30

hemisphere in dyslexics from all three countries with the maximum peak in the middle temporal

gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus

Their study showed that there was a universal neuro-cognitive basis for dyslexia and that

differences in reading performance among dyslexics of different countries are due to different

orthographies Languages with transparent or shallow orthography (such as Italian) the letters

of the alphabet alone or in combination are in most instances uniquely mapped to each of the

speech sounds occurring in the language Learning to read in such languages is easier than in

languages with deep orthography (such as English and French) where the mapping between

letters speech sounds and whole-word sounds is often highly ambiguous 71

125 TreatmentIntervention

Individualized instruction of dyslexics emphasizing increased phonologic awareness can have a

favorable long-term effect on academic achievement Individualized instruction aimed at

increasing phonologic awareness decoding skills sight word vocabulary and reading

comprehension are particularly helpful to school aged children experiencing difficulties with

reading Most importantly intervention (provided by expert teachers) should be initiated early50

Reading disabilities require lifelong assistance and optimal management strategies differ

depending on the patientrsquos age and circumstances In early childhood the focus is on

remediation of reading often with an emphasis on increasing phonologic awareness Other

strategies include using audio books and modified homework assignments For secondary and

college students intervention focuses on accommodations which include extra time for reading

tape recorders in the classroom audio books instruction in word processing and the use of a

spell-checker (poor phonemic association also causes problems in spelling)3450

126 Role of Optometry in the Multidisciplinary Management of

Dyslexia

The role of the optometrist in the multidisciplinary management of dyslexia starts with an attempt

31

to identify characteristics which may indicate that the child may be dyslexic A complete visual

examination should be conducted Any vision anomalies detected should be corrected and a

prompt referral should be made to the appropriate specialist in the diagnosis of dyslexia and a

follow up should be made by the optometrist

A comprehensive visual evaluation should include1 72

1 Evaluation for a sensory problem or refractive anomaly

2 Evaluation for binocular vision problems

3 Evaluation for ocular pathology

4 An evaluation of visual motor skills (required for drawing and handwriting) eye movement

control skills and visual imagery skills

5 Evaluation of visual information processing including visual spatial skills

(rightleft discrimination) visual analysis skills (matching and discrimination skills)

The role of the optometrist in the management of reading dysfunctions is to identify existing

vision disorders which can be achieved by evaluating the following aspect of vision functions

1261 Case History

A detailed case history is paramount for children who present with reading dysfunction A

thorough investigation of the pre-natal peri-natal and post-natal risk factors is important to

establish the possibility of major developmental delay For example maternal history of

alcoholism may indicate foetal alcohol syndrome Prematurity (early birth date of less than 37

weeks or low birth weight of less than 2500g) is also a factor in later learning A working

knowledge that may not necessarily mean expertise in all related areas may be adequate for the

optometrist72

1262 Refractive and Binocular Functions

A thorough investigation and compensation for refractive and binocular vision functions should

be undertaken4472

1263 Visual-Motor Skills

32

VisualndashMotor integration is the ability to coordinate visual information-processing skills with

motor skills44 It is a very vital skill in academic achievement4472 Children suffering from visual-

motor dysfunction have difficulty copying written work accurately and efficiently Difficulty

with copying can reduce the speed and accuracy of writing which subsequently affects

learning4472 The Beery Test for Visual-motor Integration is particularly useful for the primary

care optometrist It requires only a few minutes to administer with an added advantage that the

scoring guide provides many examples of the pass-fail criterion for each item tested72

1264 Visual-Spatial Analysis

ldquoThe visual-spatial skills allow the child to make judgments about location of objects in visual

space in reference to other objects and to the individuals own body It develops from awareness

within the individualrsquos body of concepts such as left and right up and down and front and back It is important for good motor coordination balance and directional sensesrdquo 72 The Rosnerrsquos

Visual Analysis Skills programme is useful for the assessment of both visual-motor skill and

spatial analysis72

1265 Short- Term Visual Memory (Visual-Motor recall)

The visual memory test assesses the ability to recognize and recall visually presented material 4473 The ability to copy written material accurately is more or less a matching process when the

written materials are close (such as copying from workbook to paper) When the reading

material is separated from the actual visual-motor task such as copying from chalkboard to

paper the process becomes more complex An example of short-term visual memory skill

assessment test is the Getman-Henderson-Marcus Test of Visual recall72

1266 Auditory Analysis Skills

Screening for auditory-perceptual skill at the primary care level is essential as the ability to

analyze spoken language into separate sounds and sound sequence is directly related to reading

and spelling success Examples of tests to measure auditory perceptual skills are the Test of

33

Auditory Analysis Skills (TAAS) (which is easy to administer and score) and a more complex

one Test for Auditory-Perceptual Skills72

127 Optometric Treatment Options

The optometric treatment options for the correction of vision defects include the following

options

1271 Correction of Vision Defects

1 Compensation for refractive error

2 Treatment of visual efficiency deficits with lenses prisms and vision therapy

3 Treament of vision information processing deficits with vision therapy commencing with

visual spatial orientation then visual analysis and concluding with visual-motor

integration

4 Referral to another health care professional educational system and or psychologist

should be considered at any time if underlying physical or neurological problems cognitive

deficits or emotional disorders are suspected1 50 72

The expected outcome of an optometric intervention is an improvement in visual function with

the reduction of vision anomalies associated with reading which makes educational

intervention easy1 Solan47 proposed that the role of optometrists should not end with

correcting vision anomalies in dyslexic patients but that optometrists should participate in

community services to assist with the developmental environment of those whose genetic

make-up may predispose them to having reading difficulties

1272 The Use of Tinted Lenses and Coloured Overlays

The patients rely on professionals to attend to their vision care needs Consequently it is

imperative that we practice at the highest level of professionalism and ensure that anything we

prescribe for our patients are supported with credible research

As reported by Evans74 75 Olive Meares in 1980 suggested that some childrenrsquos perception of

text and certain cases of reading difficulties were influenced by print characteristics which

34

subsequently resulted in visual perceptual anomalies such as words blurring doubling and

jumping Subsequently Meares and Irlen proposed that ldquoScotopic sensitivity syndrome or

(Meares -Irlen syndrome) ndash a syndrome of visual symptoms and distortion can be alleviated

with coloured filters Meares and Irlen claimed that the coloured overlays improve reading

ability and visual perception increase sustained reading time and eliminate symptoms

associated with reading such as light sensitivity eyestrain headaches blurring of print loss of

place and watery eyes75 This type of symptoms is reported both by people who experience

frequent severe headaches of the migraine type as well as some people with dyslexia76

Following this initiative Irlen developed a proprietary system in countries like the United States

of America United Kingdom and Australia to detect and treat Meares-Irlen syndrome Meares-

Irlen syndrome is reported to be treated with coloured filters either coloured sheets (overlays)

placed on the page or colored lenses Computer users may attain a similar effect by changing the

colour of the screen background and or text while people that consistently work under the same

lighting conditions can benefit from the colour therapy by simply varying the colour of the

illuminating light75

The earlier studies of Irlen was criticized for its lack of published double masked studies with a

placebo control to support her claims that the colour that helped had to be specific for each

person76

According to Lightstone76 the Intuitive Colorimeter (developed by Professor Wilkins) of the

Medical Research Council in the United Kingdom was the first to be used to conduct a masked

trial With this instrument it was possible to determine the optimum colour for the spectacle lens

without placing lenses in front of the eyes and could be done in a way that the subject is unaware

of the exact colour they had selected At the end of the study it was felt that the use of colour

was credible and was then included as part of a routine assessment of people with specific

learning difficulties at the Institute of Optometry London76

As described by Lightstone76 the Intuitive colorimeter is basically a large box which the patient

or subject looks into The space that the subject looks at is entirely diffused by a coloured light

and so creates the effect of wearing coloured lenses There are no other colours in the field of

35

view and so no reference colours are available to help the subject decide the exact hue (colour) of

the light that they are viewing This encourages rapid colour adaptation The person operating

the colorimeter changes all aspects of the colour that the subject is viewing and the hue

saturation (depth of colour) and the brightness of the colour can be varied independently of each

other Although the procedure is subjective the technique shows whether there is a specific

colour that gives relief from the symptoms If the effect is placebo the results tend to be variable

and no specific colour can be selected as the optimum colour75 76

An advantage of the Intuitive colorimeter is that because of colour adaptation the person is

unaware of the exact color of the light shining on the text74 However a major demerit with

double-masked trial was that the results were purely subjective and there were no objective

measurement of any improvement with the colour 76

128 Potential Mechanism for Benefit from Tinted Lenses

Some theories have been proposed to be the mechanism of benefit from tinted lenses However

Evans 49 suggested that these hypotheses have not been able to account for the high degree of

specificity of the required colour which has been emphasized by Irlen and substantiated by

double-masked randomized placebo-controlled trials

The theories are

a Pattern Glare The basis of the theory on how pattern glare explains the benefits of tinted

lenses is that certain cells within the visual cortex are hypersensitive and so causes

discomfort while viewing patterns of repeating black and white lines on a page 75 Since

some of the neurons in the visual cortex are sensitive to specific colours varying the colour

of the illuminating light may change the pattern of excitation within the cortex This could

account for the benefit from specific coloured filters From this it has been hypothesized

that by changing the colour of the input the stimulus is transmitted to other cells that are

not hypersensitive and so prevent the distortions and discomfort occurring 7576

b Syntonics Syntonics is a colour vision therapy system used by some optometrist for the

treatment of several conditions including strabismus amblyopia and reading disorders

36

The therapy involves subjects viewing a light source covered by a colored filter The theory

is that the colour influences the endocrine system through the inferior accessory optic tract

The colour seems to be chosen on the basis of a hypothetical connection between an alleged

autonomic nervous system imbalance and the type of heterophoria Treatment is said to

result in improved visual fields7475

c Placebo Effect The placebo effect is one of the possible mechanisms for the benefit from

tinted lenses People may believe that a treatment works simply because it is claimed to be

effective74

Williams and Kitchner77 conducted a literature review on the use of tinted lenses and colored

overlays for the treatment of dyslexia and other related reading and learning disorders and

concluded that

1 there was evidence that the symptoms associated with Irlen syndrome are related to

identifiable vision problems such as binocular and oculomotor problems and that condition

returned to normal when treated with lenses prisms or vision therapy When patients

exhibiting the Irlen syndrome were treated with vision therapy the symptoms were relieved

and therefore did not demonstrate the need for colored lenses

2 most investigators did not control for the presence of vision anomalies

3 the results of prospective controlled research on the effectiveness of tinted lenses or colored

overlays varied

4 the results of testing used to determine the most appropriate color were not repeatable

5 the effects of spectral filters and colored overlays were not solely a placebo Colored

overlays and tinted lenses are not cures for dyslexia but may be useful reading aids for

some individuals with reading difficulty

6 there was a lack of agreement about the best ways to evaluate patients for the presence of

Irlen syndrome

The American Optometric Association78 issued a policy statement on the use of tinted lenses and

colored overlays which states as follows

37

1 Undetected vision problems may be a factor in individuals who exhibit the symptoms of

the Irlen Syndrome A comprehensive eyevision examination with particular emphasis

on accommodation binocular vision and ocular motor function is recommended for all

individuals experiencing reading or learning difficulties as well as those showing signs

and symptoms of visual efficiency problems

2 The American Optometric Association encourages further research to investigate the

effect that specifically tinted lenses and colored overlays have on visual function related

to reading performance

3 Vision problems are a frequent factor in reading difficulties Ignoring the role of vision

or inadequately evaluating the vision of individuals with reading problems is a disservice

which may prevent the person from receiving appropriate care

In concluding Lighthouse76 advices that until more is known about prescribing specific colored

lenses optometrists should be certain to offer the patient the best advice According to Wilkins 79

some 20 of unselected children in mainstream education (not simply those with reading

difficulties) find colored overlays of benefit and the benefit is pronounced in 5 Wilkins79

emphasized that ldquocolored filters do not provide a treatment for dyslexia ndash they provide a means of

avoiding the visual stress with which reading is sometimes associatedrdquo

129 SUMMARY

Dyslexia is a neuro-developomental condition that impacts on a persons spelling

comprehension writing and reading and not only limits their academic career but also impacts

on all areas of life that require these skills Its diagnosis management and treatment require a

multidisciplinary team as it can present with many characteristics which requires careful

classification While it is not always possible to identify the cause it is generally considered to

be a neurological condition with a genetic origin It affects up to one fifth of Caucasians with

limited studies being done on other ethnic groups including Africans

The aim of the study was therefore to investigate the prevalence of vision defects in an African

38

South African population of dyslexic schoolchildren with an attempt to identify high risk

visual conditions that may be prevalent It was hypothesized that there was no relationship

between vision defects and dyslexia

The reading process requires learning to transform printed letters on a page to words that have

meaning Words are made up of phenomes which are represented by letters and it is the

inability to break words into parts that result in the manifestation of dyslexia Its characteristics

include letter reversal faulty pronunciation difficulty with word retrieval and learning to talk

As all visual stimuli is transmitted through the eye there is an association between ocular

conditions and problems associated with letter words and reading hence the involvement of

optometrists The primary role of optometry is to identify and compensate for visual defects

that may constitute an impediment to reading Evaluation by an optometrist is therefore

important to assess and provide advice or devices where appropriate

Initial identification usually takes place by teachers or parents and may result in the child

being sent to a school equipped to deal with learning disabilities Psychologists diagnose

dyslexia using some range of psychometric tests The advent of imaging techniques such as

positron emission tomography and functional magnetic resonance has aided research in

dyslexia as these techniques enable the study of dyslexic brain activity as a reading task is

performed The two broad categories of dyslexia one being due to a genetic neurological

defect and the other being due to a secondary incident that impacts on the brain There are a

number of theories regarding the underlying causes the only theory related (still controversial)

to visual system being the visual processing theory

Chapter One provided the study aims objectives hypotheses to be tested background

information on dyslexia and rationale for the study while Chapter two will review the related

literature on dyslexia and vision

39

CHAPTER TWO LITERATURE REVIEW

21 Introduction

The relationship between vision and dyslexia has been a subject of controversy in optometric

and ophthalmologic literature A review of existing literature 1-32 on the area of vision and

dyslexia reveals that the studies conducted to investigate visual functions in the dyslexic

subjects do not establish a causal relationship between vision and dyslexia but tend to

establish associations between dyslexia and certain vision variables The literature review also

shows that there exists a dichotomy between the ophthalmologists and optometrists on the

subject of dyslexia and vision The difference in perspective can be seen from the style of

report in studies conducted by optometrists 5683-849198100-101 and by ophthalmologists 2-4 85 95

99102 Optometrists maintain that although vision may not be considered as a cause of dyslexia

compensating for visual defects make reading easier for the dyslexic child Ophthalmologists

hold the contrary view that vision does not in any way contribute to dyslexia Dyslexia has

also been thought to be a language-based learning disability80 Consequently from the speech-

language pathologistsrsquo perspective ldquoa child with a speech deficit may also have an emotionally

distorted perspective and a child whose classroom behaviour is hard to control may also be

unable to readrdquo The role of the psychologists is to perform psychometric assessments of the

dyslexic child32 The educator is primarily involved with special educational intervention for

the dyslexic child

Flax 22 proposed that the role of visual factors in dyslexia ldquodepends on the definition of vision

that is consideredPrime meaning that if vision is narrowly defined as visual acuity or clarity of

sight it is unlikely that there would be any relationship between vision and dyslexia If other

peripheral visual functions such as fusion convergence refractive errors and accommodation

are considered there is likely to be a relationship but then questioned the possibility of vision

being a causative factor Flax22 reiterated that the most important aspect of vision that is

closely linked to dyslexia is when vision is defined to include perceptual and integrative

aspects of vision It was concluded that ldquovision difficulties are part of the dyslexic

40

syndrome22

From a similar perspective Solan81 suggested that vision anomalies are contributory and

frequently hinder responses to educational intervention He emphasized that in cases where

vision anomalies are factors involved in the reading disability the effort required to maintain

clear single binocular vision decreases the efficient organization of the visual cognitive

response

This review has been structured in a way that an overview of a study will be made when it is

first cited Subsequently such studies will be cited only with references to the vision variable

being reviewed Only the studies that relate directly to the topic of the present research such as

visual acuity refractive errors near point of convergence ocular alignments accommodation

functions and vergence reserves are reviewed

It is also important to note that there are many variations in the results reported by different

authors In view of this the findings are thus presented on an individual basis Various studies

have attempted to study the relationship between dyslexia and vision and have examined the

different aspects of vision suspected of affecting reading ability However the major

drawbacks that plague the researches in the subject are Firstly intelligence quotient is a factor

to consider when defining dyslexia However only a few studies587-89 consulted have

considered intelligent quotient in their subjectsrsquo selection

Another area worth mentioning is the fact that some of the studies lacked control group which

made the reports difficult to assess and utilize In addition some researchers failed to classify

some visual functions such as binocular coordination and refractive errors thereby making it

difficult to evaluate the impact of a particular vision variable on reading ability Furthermore

some studies failed to indicate the eye examination techniques used in their protocol which

makes the replication of such studies difficultGiven the inherent limitations with studies

conducted to investigate vision in dyslexic children these studies are important in guiding the

teacher and clinicians on the appropriate intervention for the child

22 Visual Acuity

41

Vision plays an important role in the reading process as the ability to see printed material is

crucial in reading Reading efficiency and speed starts to decrease when the print size is less

than three times larger than an individualrsquos visual acuity threshold A marked bilateral

reduction in visual acuity can definitely limit reading performance but it is unknown if

deficient visual acuity characterizes students who have difficulty to read82

Available studies3-58384 that have attempted to see how visual acuity affects reading

performance are presented starting with the first three studies in this review that found a higher

prevalence of reduced visual acuity among dyslexics compared to normal readers

In 1994 Latvala Korhoenen and Penttinen 3 assessed the ophthalmic status of fifty five (55)

dyslexic and fifty (50) control subjects in an age gender and social class matched study in

Finland The subjectsrsquo school grade and intelligence quotient were not indicated Since

intelligence quotient is a major factor in the definition of dyslexia the lack of information on

intelligence quotient may lead to the questioning as to if the population studied were truly

dyslexics The authors assessed visual acuity using the Snellen chart and found that for the

control group all subjects had visual acuity of 07 (69) while in the dyslexic group two

children (36) had bilateral visual acuity of less than or equal to 07

A study to examine the lsquoophthalmologicalrsquo status of school children with dyslexia in Norway

was conducted by Aasved 4 in 1987 Visual examinations were performed on two hundred and

fifty nine dyslexic children The number of the participants from the control group was not

clearly stated The investigation techniques used was also not detailed As a result it may be

impossible for other researchers to replicate the study However Aasved 4 reported that the

dyslexic population had a higher prevalence of reduced visual acuity than the normal readers

did It was concluded that although there was no causal relationship between eye

characteristics and reading difficulties eye abnormalities should be corrected when detected in

dyslexic children

42

Evans et al 5 in 1993 studied some sensory and refractive visual factors in 39 dyslexic and 43

control subjects aged between seven years six months and twelve years three months in the

United Kingdom Participants from both groups had intelligence quotient of above 85 The

authors found that the dyslexic group had a significantly worse near and distance binocular

visual acuity the inter-ocular difference was not statistically different in the two groups and

the difference between the best monocular and the binocular visual acuity was not statistically

different between the two groups at distance or near They claimed that the ldquoreduced visual

acuity may not have been due to refractive defectsPrime Apart from refractive errors a reduced

visual acuity may arise from pathological conditions such as cataract and glaucoma

The following two studies reported similar prevalence of visual acuity worse than 69 In 1980

Hoffman83 investigated the incidence of vision difficulties in learning disabled children in the

United States of America Vision evaluations were performed on one hundred and seven

children with learning problems who were referred to the clinic for vision care from educators

and psychologists The subjectsrsquo mean age was eight years eight months As a comparison

(control) group twenty five patients between the ages of five and fourteen were selected from

the general optometry clinic The prevalence of visual acuity poorer than 69 was reported to

be 1028 among the learning disabled subjects The subjects from the control group were not

appropriately selected not representative enough and were not age-matched although the issue

of the comparison group being inappropriately selected was acknowledged by the author

Another limitation with the study was that no information was given on the eye examination

procedures followed This may be problematic as it makes comparison with other studies and

replication of the study difficult

Sherman84 conducted a study to ldquorelate vision disorders to learning disabilitiesrdquo in the United

States of America in 1973 Thirty-nine boys and eleven girls participated in the study The

children were diagnosed as having learning difficulties by the referring psychologists Their

43

ages ranged from six to thirteen years The technique used to measure visual acuity was not

indicated but it was reported that five children (10) had visual acuity poorer than 69

Other studies reported the prevalence of visual acuity to be similar between the control and

reading disabled group and include the following studies In 1985 Helveston Weber and

Miller85 (from ophthalmology) studied visual functions and academic performance in first

second and third grade children in the United States of America One thousand nine hundred

and ten children participated in this study The Snellen chart and the Jaegar print were used to

assess distance and near visual acuity respectively The authors reported that 94 of the

children tested had normal visual acuity and that visual acuity was not related to academic

performance in a positive way Although the study may be credited with a large sample size a

possible limitation with the application of the findings of this study was that the passfail

criteria were arbitrarily chosen by the examiners ldquoon the basis of previous clinical experiencerdquo

without references to other research studies Secondly the authors failed to present the results

of the statistical analyses of the relationships between vision functions and reading

Grisham and Simon82 conducted a review of literature on refractive error and reading process

and reported that there are abundant and reliable scientific evidence that shows a higher

prevalence of vision defects among reading disabled children than among normal readers It

was emphasized that there are cases where prescription of glasses has improved academic

performance Their analysis revealed no statistically significant difference in distance visual

acuity among normal and disabled readers but that near visual acuity may be an index which

distinguishes some children who are having difficulty to read and those who do not

It is important to note that since reading is a near point task a deficient near visual acuity may

be more contributory to poor reading than reduced distance acuity Consequently Grisham

and Simons82 stated that ldquodeficient distance visual acuity would not be expected to affect the

44

reading skills of children unless substantial reading instructions take place on the chalk board

It is possible that distance acuity is a factor in the early grades when reading is first

introducedrdquo

In 2007 Grisham Powers and Riles86 studied visual skills of 461 poor readers in California

high schools The participants mean age was 154 years and there was no control group in the

study Participants for the study had been identified by the schools as poor readers The vision

functions measured were visual acuity near point of convergence ldquoconvergence and

divergence and fusional rangesrdquo accommodative amplitude and accommodative facility No

information was given on IQ but it was mentioned that the basis for referral of students for

assessment was poor reading performance which was determined by the school and defined as

reading two grade levels or more below grade level Visual acuity was assessed using the

Snellenrsquos chart at six meters The authors reported that 568 of the participants had visual

acuity of 66 in both eyes 261 had visual acuity of 69 in each eye while 172 had visual

acuity of worse than 69 in either eye

Another comprehensive study conducted in 2007 was reported by Kapoula Bucci and

Jurion87 Convergence and divergence functions in 57 dyslexic and 46 non-dyslexic age-

matched children were studied in French dyslexic children The mean age of the dyslexic

subjects was 113 years the mean IQ was 105 and the mean reading age was 891 The mean

age for the control group was 107 years The IQ score of the control subjects was not

indicated The vision functions performed included visual acuity stereo acuity and cover test

near point of convergence and ldquomeasurement of convergence fusional amplitude (divergent or

convergent) Unfortunately the authors did not detail the testing procedures for the vision

functions investigated but reported the results of the study It was reported that all children had

normal visual acuity

In 2008 Alvarez and Puell 88 studied accommodation functions in school children with reading

difficulties in Madrid Spain Eighty seven poor readers (mean age 92 years) and 32 control

(mean age not indicated) participated in the study all in grades three through grade six

Relative accommodation amplitude of accommodation and accommodation facility was

assessed

45

The most current literature reviewed was reported by Bucci Gignac and Kapoula 89 in 2008

The authors studied poor binocular coordination of saccades in dyslexic children in France Of

particular interest to the current review is the assessment of visual acuity and refraction

(though was assessed under cycloplegia) heterophoria near point of convergence and fusional

vergence reserves The study participants comprised of 18 dyslexic children (mean age 114

years and mean IQ was 105) The control group comprised aged-matched non-dyslexic

children mean age 11 2 years There were no details on the method of assessment of visual

acuity and refraction but it was indicated that a cycloplegic refraction was conducted

However the authors reported that all children had normal visual acuity but the results for

refractive errors were not reported

In 1991 a study on stereopsis accommodative and vergence facility was conducted by

Buzzelli 6 in the United States of America Thirteen normal readers of average age thirteen

years three months and thirteen dyslexics average age of thirteen years four months

participated in the study Both groups were matched for gender age and intelligence quotient

The study design was comprehensive The Snellen chart was used to assess the visual acuity

Buzzelli6 found that the dyslexics did not perform worse than the control subjects in visual

acuity

Ygge Lennerstarnd Axelsson and Rydberg2 studied visual functions in a Swedish population

of dyslexic and normally reading children in 1993 Eighty six 9-year- old children matched to

controls with regard to age gender class in school and intelligence participated in the study

VA was assessed with ldquoordinary optotypes chartsrdquo They reported that the subjects from the

control group had a better visual acuity than the dyslexic group at both distance and near and

that the results showed statistically significant differences (distance at p =003) and (near at

p=0005)

A comparative study of dyslexic and normal readers using orthoptic assessment procedures

was conducted by Goulandris McIntyre Snowling Bethel and Lee90 The study was

conducted in 1998 in the United Kingdom and comprised 20 dyslexic and 20 chronological and

46

reading age-matched control participants A unique aspect of the study was that the distance

VA was assessed using the Cambridge Crowding Cards This test according to the authors

was devised to evaluate letter recognition acuity in conditions of lsquocrowdingrsquo when other letters

surround the target letter and that the test is designed to ensure that abnormalities of vision

which would not be revealed reliably using single targets were detected The near acuity was

assessed using the Snellenrsquos chart The use of different VA chart at distance and near may

raise a concern of standardization They reported that the prevalence of distance and near VA

(using a 69 criteria) was similar between the groups

In 1992 Evans Drasdo and Richards91 investigated optometric correlates of reading disability

in 10 children aged between eight and fifteen years who were referred to the optometry clinic

by the child psychologist The authors reported only the statistical values mean binocular

distance visual acuity of + 091 and mean binocular near visual acuity of + 089

47

23 Refractive Errors

In the study by Alvarez and Puell 88 no information was given on the protocol used to assess

refractive error but the refractive error results were reported For the poor readers mean

spherical equivalent refractive errors for the right and left eye were 020 06 and 020 06

respectively Fifty-seven (655) children were emmetropic In the control group mean

spherical equivalent refractive errors for the right and left eye were - 020 08 and - 014

080 respectively Twenty (645) children were emmetropic In contrast Evans et al 91

reported the mean spherical equivalent refractive error (by retinoscopy) of + 077DS for the

(right and left eye was similar)

231 Myopia

Myopia is characterized by a decrease in distance visual acuity Myopia would usually not be

expected to affect reading skills since students can make the necessary adjustments to their

reading positions However from a clinical standpoint unusually high myopia myopic

anisometropia and astigmatism could influence patients near performance and cause some

discomfort to the child There is no evidence in the literature that myopia is associated with

poor reading In the study by Alvarez and Puell88 57 of the children from the dyslexic

group were myopic compared to 194 from the control group

In 1987 Rosner and Rosner 93 conducted a retrospective comparison of visual characteristics of

data for two hundred and sixty one children with learning difficulties in the United States of

Ameria who had been identified by the schools as manifesting learning difficulties (LD) The

control group consisted of four hundred and ninety six children without learning difficulties

The subjects from both groups were of age range between six and twelve years The children

had been examined at the University of Houston Optometry Clinic The data collection

techniques used was not given but it may be assumed to be routine optometric eye examination

48

protocols because the study was conducted at the university optometry clinic The prevalence

of myopia (minus 050 dioptres) was reported to be 54 in the non-learning disabled patients

while 19 of the learning disabled patients were myopic

One limitation with this study however was that all subjects were patients of an eye clinic

where the two main reasons for referral are reduced visual acuity and school learning

problems Consequently the prevalence of visual defects in subjects selected from such a

clinical setting may have been higher than those from a non-clinical setting

In a review study conducted by Grisham and Simon82 the prevalence of myopia was lower in

the reading disabled group than among the normal readers Similarly Grosvenor 94 analysed

data from several studies and concluded that myopia is consistently associated with good

reading performance One limitation with this analysis however that is the conclusion drawn

from the review was based only on studies that found positive relationships between vision

anomalies and reading ability Grosvenor 94 however remarked that ldquomany authors failed to

find the relationship because they used ineffective methods

One of the earliest studies on the subject of vision and reading difficulty was conducted by

Eames 95 in 1948 in the United States of America He studied refractive and binocular vision

anomalies in one thousand (1000)rdquoreading failures one hundred and fifty (150) unselected

children and five hundred ophthalmic patientsrdquo Comparing the poor readers and the

unselected groups (control) he found the incidence of myopia to be same in both groups (4)

Myopia was defined as -100 diopter sphere There was no statistical analysis in that study and

the data collection techniques were vague although it was indicated that no cycloplegia was

used While the study may be criticized as not being current it is the major and early study to

give an insight into the subject of vision and reading disability

Eames95 and Shearer96 reported a similar prevalence of myopia (4) Shearer96 investigated

ldquoeye findings in children with reading difficultiesrdquo Two hundred and twenty two children

participated in the study The subjects were screened using the Keystone telebinoculars The

prevalence of myopia defined as - 050 dioptres sphere was 4 He concluded that vision

49

functions are not related to reading ability

232 Hyperopia

Hyperopia is often associated with poor reading performance probably through the mechanism

of deficient accommodation and poor motor fusion skills 82 A child with a relatively high

degree of farsightedness may not report blur vision at distance due to compensation for

hyperopia through the accommodation mechanism which also is used to create clear vision at

near This results in overuse of the accommodation system both optically and physiologically

and a subsequent fatigue or spasm of the accommodation system 97

One of the studies that classified refractive error in the literature was conducted by Rosner and

Rosner93 They reported that hyperopia appeared to be more prevalent (54) in learning

disabled than in the non-learning disabled children with a prevalence of 16

In the study conducted by Alvarez and Puell88 287 of the dyslexic group was hyperopic

compared to 161 from the control group Grisham and Simon82 in a literature analysis of

refractive errors and reading process reported that the prevalence of hyperopia among poor

readers is higher than among good readers and that the correction of hyperopia resulted in

improved performance

Grosvenor 94 reported that hyperopia is usually associated with poor reading Similarly in

Eamesrsquo study95 43 of poor readers had hyperopia while for the control group only 12 were

hyperopic The author gave no information on statistical analysis Shearer96 reported the

prevalence of hyperopia to be 16

233 Astigmatism

Clinically uncorrected moderate to high astigmatism can cause discomfort and lead to poor

reading performance at near distance Relatively few studies have attempted to study the

relationship between astigmatism and reading disability The prevalence of astigmatism

reported was higher in the dyslexic group compared to the control group in all the studies

reviewed 39395 These studies include the study by Latvala et al 3 who reported that 36 of

50

the dyslexic subjects were astigmatic (of greater than -100 diopter cylinder) and none from the

control group had astigmatism of that magnitude Eames95 reported the prevalence of

hyperopic astigmatism of more than -100 diopter cylinder among the poor readers to be 6

and 4 in the unselected group while the prevalence of myopic astigmatism was the same

(1) in both groups Rosner and Rosner 93 found a higher prevalence (30) of astigmatism

amongst the non-learning disabled subjects compared to the learning disabled group (27) In

Shearerrsquos 96 study of eye findings in children with reading difficulty the total prevalence of

astigmatism classified as myopic astigmatism (3) and hyperopic astigmatism (3) reported

was 6 There was no control group in this study In the study by Alvarez and Puell 88

astigmatism was detected in seven (8) and eight (92) right and left eyes respectively while

astigmatism was detected in five children (161) from the control group Ygge et al 2

reported that the prevalence of astigmatism was higher in the dyslexic group (28 left eye 25

) compared to the control groups (right eye 183 left eye 243) although there was no

statistically significant difference between the groups (p=025)

234 Amblyopia

Only two studies on the available literature consulted reported on the prevalence of amblyopia

both indicating a higher prevalence amongst the dyslexic group compared to the control group

In the study by Latvala et al 3 the prevalence of amblyopia in the dyslexic group was 36 and

none in the control group had amblyopia Although a detailed prevalence of refractive error

was not presented in that study it was reported that nearly all subjects had visual acuity of 07

In relating refractive error to amblyopia the prevalence of astigmatism in the control group

was 36 and none from the dyslexic group had astigmatism Consequently a low prevalence

of amblyopia may be related to fairly good visual acuities in both groups On the contrary

Rosner and Rosner 93 reported that 4 of the non-learning disabled patients had amblyopia

compared to the 3 found in the learning disabled patients Hyperopia is the refractive error

that is more often associated with amblyopia However this study presented a higher

prevalence of amblyopia (4) but a lower prevalence of hyperopia (16) in the non-learning

disabled patients However a difference of only 1 is not high enough to account for the

difference

51

235 Anisometropia

Anisometropia has been thought to be associated with poor reading skills probably through the

mechanism of poor sensory and motor fusion rather than reduced visual acuity It degrades

binocular coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 82

Three studies2395 reported findings on anisometropia Latvala et al 3 reported a higher

prevalence of anisometropia (of greater than or equal to one diopter) of 6 for the control

group than in the dyslexic group with 36 Similarly Ygge et al 2 found a higher prevalence

of anisometropia in the control group (158) than in the dyslexic group (94) Eames 95

reported that 13 of poor readers were anisometropic while 6 from the control group had

anisometropia According to Grisham and Simons 82 only few studies have considered

anisometropia to affect reading performance The authors concluded that there was insufficient

number of studies to draw firm conclusions from their review

Other studies2598 failed to classify refractive error according to the type These include

studies by Evans et al 5 who investigated some sensory and refractive factors in dyslexia The

distribution of refractive errors was found to be similar in both the dyslexic and control groups

The details of the participants profile was given earlier (section 21) The refractive errors

were assessed by distance retinoscopy and subjective refraction A cross cylinder was used to

measure astigmatism The spherical equivalent refraction was calculated by adding one-half

of the cylindrical components to the spherical component It was not indicated if refraction

was performed under cycloplegia Ygge et al 2 assessed refractive errors objectively using

streak retinoscopy under cycloplegia They reported that the distribution of refractive errors

were similar between the dyslexic and control groups

Sucher and Stewart 98 examined 136 fifth and sixth grade children comprising seventy two

with learning disability and 64 from the mainstream school as the control group The learning

52

disabled children were selected from a classroom of children attending special supervision and

instruction Children from both groups were matched by grade level No information on

intelligence quotient was given The testing protocol used to assess refractive error was not

indicated The authors reported a higher prevalence of refractive error in the dyslexic (208)

than in the control (94) group As indicated earlier refractive errors were not classified

according to the types in the three preceding studies 2598

Hoffman 83 reported the prevalence of refractive error to be 215 There was no report on the

control group and the techniques used to assess refractive errors were not indicated In the

study by Helveston et al 85 only 1 of the participants had refractive error when assessed

objectively with the retinoscope without the use of cycloplegia Refractive errors were not

classified according to the types (myopia hyperopia or astigmatism)

In 1991 Hall and Wick 99 conducted a study to investigate the relationship between ocular

functions and reading achievement in the United States of America One hundred and eleven

children (in grades one through grade six) participated in the study Eleven ocular functions

were investigated including visual acuity saccade eye movements accommodation facility

amplitude of accommodation accommodation posture heterophoria horizontal fixation

disparity vertical fixation disparity near point of convergence titmus and Randot stereopsis

Accommodation facility was measured using the flipper technique The amplitude of

accommodation was determined using the push up to blur technique Heterophoria fixation

disparity and near point of convergence were also measured but the techniques used were not

indicated The age range of the subjects studied was not indicated Using multivariate

correlation they found no statistically significant relationship between the ocular functions and

reading abilities as they noted that the acquisition of reading skills is given by many forces

which include the use of remedial instructions and language skills the role of peer pressure and

innate intelligence

53

24 Near Point of Convergence

Anomalies of convergence is one of the common problems encountered in the optometric

practice especially in the pediatric population and the reports on near point of convergence has

been presented comprehensively in the literatures reviewed

Grisham et al 86 measured the near point of convergence by asking the student if the clown face

target was seen as single when the examiner held it approximately 20 cm in front of the

studentrsquos eyes If so then the student was requested to say when the object appeared to double

as it moved closer to the face The examiner placed one end of an accommodative rule on the

studentrsquos forehead and judged where the two eyes diverged as the target moved toward the

studentrsquos face The measurement was repeated three times The authors found that 846 of

the students had near points at 8 cm or closer which they considered ldquonormalrdquo while 154

had near point of convergence of 9 cm or farther

In the study by Kapoula et al87 the NPC was determined by placing a small pen-light at

3040cm in the mid plane in front of the subject and moving it slowly towards the eyes until

one eye lost fixation The authors reported that the NPC was significantly more remote

(gt10cm) in dyslexics than in non-dyslexics (36 versus 15)

Bucci Gignac and Kapoula 89 followed similar protocol as in the study by conducted Kapoula

et al 87 The near point of convergence was determined by placing a small pen-light at 3040

cm in the mid plane in front of the subject and moving it slowly towards the eyes until one eye

lost fixation It was reported that the NPC was not statistically different between the two

populations the median value was 5 cm for both dyslexic and non-dyslexic children

Latvala et al3 failed to indicate the technique used in assessing the near point of convergence

54

but simply noted that ldquoconvergence ability was measuredrdquo but reported that convergence near

point greater than or equal to 8cm was found in 127 of the dyslexic subjects and 2 of the

control group The authors concluded that the near point of convergence was the only variable

that showed a statistically significant difference between the groups They concluded that

ophthalmic factors should not be overlooked as a contributing factor to dyslexia as they may

constitute part of the dyslexic syndrome and recommended that vision anomalies should be

corrected whenever detected

Shearerrsquos 96 findings are similar to the results reported by Latvala et al 3 He assessed the near

point of convergence using the keystone telebinoculars and reported that 12 of the

participants in his study had convergence problems

In a relatively elaborate study Evans Drasdo and Richards100 investigated accommodative and

binocular functions in dyslexia The subjects were 43 control and 39 dyslexics aged between

seven years six months and twelve years three months An intelligence quotient range of over

85 for the dyslexics and over 90 for the control with the Wechsler Intelligence Scale for

Children (Revised) (WISC-R) was recorded They measured the near point of convergence

using the royal air force (RAF) rule It was concluded that all subjects were able to report a

clear subjective break and recovery and reported no statistically significant difference between

the two groups in either the near point of convergence

Hoffman83 assessed convergence using keystone visual skills tests but grouped the results as

binocular coordination which makes it difficult to analyze and compare while in the study by

Helveston et al 85 98 of the participants had normal near point of convergence

In 2002 Keily Crewther and Crewther 12 investigated the relationship between functional

55

vision and learning to read in Australian school children Two hundred and eighty-four

children (mean age 99 18 years) received a vision screening emphasizing binocular

anomalies associated with discomfort at near (distance and near visual acuity distance vision

ldquochallenged with binocular +1 D lensesrdquo near heterophoria near point of convergence

stereopsis and accommodative facility) Children were classified as normal readers (n = 195)

children with dyslexia (n = 49) or learning disabled children (children who have learning

difficulty with impaired intelligence quotient) (n = 40) It was reported that the near point of

convergence were better in the dyslexic group compared to the normal and the learning

disabled group but found no statistically significant difference between the groups In

constrast Evans et al 91 assessed near point of convergence using the RAF rule and reported a

highly statistically significant relationship between NPC and reading retardation (p=0019)

25 Heterophoria

Heterophoria is a common binocular problem and in clinical practice it is particularly

important when analyzed in conjunction with fusional reserve as it gives an indication of the

zone of comfortable binocular vision

The distance phoria is a clinical measurement of the tendency of the two eyes to deviate from

parallelism when viewing a target usually at six meters When a closer viewing target is

utilized (say at about the individuals reading distance) the resulting near phoria measures the

degree to which the eyes over converges (esophoria) or underconverges (exophoria) to the

plane of the target97 This latent deviation requires fusional vergence to maintain single

binocular vision and at near they involve the accommodation system Smooth functioning of

these systems particularly at near is important for near tasks92

Several studies312858789959698100101 have attempted to investigate the role of heterophorias on

reading ability and all reported remarkably varying findings The assessment of heterophoria

was extensive in the study conducted by Latvala et al 3 Heterophoria at distance was evaluated

56

using the cover test and Maddox rod The prism bars was used to quantify their findings The

Maddox wing was used to assess phoria at near At near they reported a higher prevalence of

esophoria (61) among the control than the dyslexic group (58) a higher prevalence of

exophoria (25) among dyslexics than the control group (122) and a higher prevalence of

vertical phoria (61) among the control than the dyslexic subjects (36) At far distance

they found a higher incidence of esophoria (113) among dyslexics than the control (82)

subjects a higher prevalence of exophoria (2) among the control group than among the

dyslexics (19) and vertical phoria to be more prevalent in the control (41) than in the

dyslexics (38)

Keily et al 12 assessed heterophoria using the Prentice card and reported that there was a slight

but insignificant tendency for the dyslexic subjects to be more exophoric than the normal and

those with learning disability but found no statistically significant difference The test distance

was not indicated

Evans et al 100 in a study of accommodative and binocular functions in 43 control and 39

dyslexics reported that all subjects examined were orthophoric at distance with cover test

except for one dyslexic subject who demonstrated 10 prism diopters right esotropia and two

diopters right hypertropia which changed to 15 prism diopters right esotropia with two prism

dioptre right hypertropia at near At near horizontal phoria results were similar in both groups

Evans Efron and Hodge 101 studied the incidence of lateral phoria in 45 children with specific

learning disability in the United States of America in 1977 The participants were selected from

a psychologistrsquos file The control group comprised of 364 children in second third and fifth

grades attending regular school It was not indicated why there was such a big difference in

the number of participants from both groups The test used in assessing phoria was the

ldquoRocket card which was used in conjunction with red- green spectaclesrdquo They reported a

57

prevalence rate of 1135 in the control group while 222 of the children from the specific

learning difficulty group had heterophoria There was no statistically significant difference

between the groups In concluding it was recommended that phoria conditions may not cause

reading or learning disability but that the presence of such conditions could contribute to visual

perceptual and attention abnormalities

A report by Sucher and Stewart98 contained limited information as the authors did not detail

whether a higher prevalence rate among control (105) than dyslexic (86) was for near or

distance phoria Similarly Eames95 failed to indicate the distance at which the measurement

was made but reported that 33 of poor readers had exophoria and 22 of control were

exophoric but did not indicate whether distance or near

Helveston et al 85 used the Wottring amblyoscope to determine ldquoobjective and subjective angle

of ocular alignmentPrime and reported that 96 of the participants in their study were orthophoric85 The criteria used to classify ocular deviation were not indicated

Another comprehensive report of heterophoria was documented by Shearer96 The subjects

were screened using the Keystone telebinoculars and reported that 1 of the study participants

had vertical phoria (gt1 prism diopter) 2 had esophoria (gt 4 prism diopter) 3 had

exophoria (gt 4 prism diopter) 26 had exophoria at near It was concluded that vision

functions are not related to reading ability Hoffman 83 reported that ninety three (93) subjects

(869) had problems of binocular coordination but failed to indicate whether it was phoria or

tropia

Kapoula et al 87 evaluated ocular alignment using the cover test and reported that at far

distance the median value of phoria was zero for both non-dyslexic and dyslexic children (p =

019) At near the median value of phoria was similar between the two groups (minus2 prism

58

diopters for non-dyslexics and minus15 prism diopters for the dyslexics (p = 019)

Bucci et al89 measured heterophoria at near and far using the cover-uncover test As

documented by the authors exophoria was not statistically different between the two groups (p

= 02) the median was ldquo0rdquo prism diopters for dyslexics and 2 prism diopters for non-dyslexics

No information was given on other aspects of heterophoria

Simons and Grisham92 conducted a literature analysis on binocular vision anomalies and

reading emphasizing studies with positive and negative relationship between binocular vision

and reading The authors concluded that binocular conditions such as esophoria exophoria

restricted ranges of fusion foveal suppression accommodative insufficiency and oculo-motor

disorders at reading range are more prevalent among poor readers than normal readers

In another study Ygge Lennerstarnd Rydberg Wijecoon and Petterson102 investigated

oculomotor functions in a Swedish population of dyslexic and normally reading children in

1993 The study comprised 86 nine -year- old dyslexic children matched to control with regard

to age gender and intelligence Heterophoria was measured using the cover test They

reported that for the dyslexic group ldquo23 pupils exhibited a phoria (264 9 eso- and 14 exo-

phoria)rdquo whereas in the control group ldquo20 pupils had a phoria (232 6 eso- and 14 exo-

phoria)rdquo and that the difference was not statistically significant (p = 034) At near the

incidence of exophoria was 437 for the dyslexic group and 453 in the control group

There was no statistically significant difference (p=055)

26 Strabismus

Several authors34 83-85 90 93102 included the measurement of strabismus in their studies

reporting remarkably varying findings Helveston et al 85 assessed ocular alignment using the

Wottring amblyoscope and reported that only between 2 - 4 of the students had phoria or

59

tropia of more than four prism dioptres Latvala et al 3 reported the prevalence of tropia at near

for the dyslexic group to be 36 and 2 for the control group Hoffman83 assessed ocular

alignment using the keystone visual skills test and reported that 13 subjects (9 exotropes and 4

esotropes) (1214) had strabismus It is difficult to comment if this relatively high

prevalence of strabismus is related to hyperopia as the refractive error prevalence of 215 was

not categorized as to the type In a study by Sherman84 a total of four children (8) were

strabismic classified as three exotropia (alternating) and one esotropia (alternating) Rosner

and Rosner93 in their retrospective analysis reported that in the learning disabled group 51

had esotropia 39 had exotropia while in the non- learning disabled group 8 were

strabismic It was not unexpected to find a higher prevalence of strabismus in this

retrospective study Aasved 4 found that the dyslexics had a higher prevalence of manifest and

latent convergent strabismus than the control subjects

Goulandris et al90 assessed strabismus using the cover test and reported that there was no

statistically significant difference between the two groups in the prevalence of strabismus

Ygge et al 102 reported that at distance manifest strabismus was more frequent among the

dyslexic (8) pupils than the controls (23) but that the difference was not statistically

significant At near the prevalence of manifest strabismus was 8 (all eso-) for the dyslexics

and 35 (all eso-) in the control group

27 Accommodation Functions

271 Amplitude of Accommodation

The accommodation mechanism is extremely important for learning Children who suffer

some anomalies of accommodation are more prone to fatigue quickly and become inattentive

than those who have normal accommodation function According to Flax 22 an ldquoinefficient

accommodation function is a significant contributor to lowered achievement in the upper

grade

60

The findings on various aspects of accommodation functions were diverse Metsing and

Ferreira 103 studied visual deficiencies in children from mainstream and learning disabled

schools in Johannesburg South Africa in 2008 One hundred and twelve (112) children from

two learning disabled schools and eighty (80) children from a mainstream school in Johannes-

burg participated in the study The vision functions examined were visual acuity refractive

status ocular health status accommodative functions (Posture facility and amplitude)

binocularity (cover test vergence facility smooth vergences NPC and ocular motilities (direct

observation) However according to the authors only the results for accommodation

accuracy amplitude of accommodation vergence facility and saccadic accuracy are reported

since only these variables showed statistically significant relationship on the relationship

between the (mainstream and learning disabled) groups Metsing and Ferreira 103 reported

that a high percentage (516 and 533) of low amplitude of accommodation for the right

and left eyes respectively was found in the mainstream group compared to the 291 and

288 in the learning disabled group The relationships between the mainstream group and

reduced amplitudes of accommodation of the right (p=004) and left eyes (p=0001) were

found to be statistically significant (plt005) The relationship between the mainstream group

and reduced amplitude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to

be moderate and that between the mainstream group and the right eyes (Cramerrsquos V= 0286) to

be low

In the study by Grisham et al86 accommodative amplitude was assessed using the

accommodative rule ldquoThe student held an occluder over the left eye The examiner asked if

the student could see the (2030) target at the larger end of the Occlud-A Measure held at about

20 cm from the face If yes then the student was asked whether the target was in focus or ldquonot

blurryrdquo If the student responded that the target was in focus then the target was moved at

approximately 2cm per second toward the studentrsquos face and the student was asked to report

when the target first became blurryrdquo About 638 of the participants had amplitude of

accommodation of 11D which the authors considered equivalent to the expected amplitude of

61

accommodation for the age About 247 had inadequate accommodation amplitude while

115 had ldquoborderlinerdquo amplitude of accommodation

In the study by Evans et al 100 the amplitude of accommodation was measured using the push up

method They reported that the amplitude of accommodation was significantly reduced in the

dyslexic group and that the mean accommodation lag was calculated for each eye but found no

statistically significant difference between the two groups

Alvarez et al 88 assessed monocular accommodative amplitude using the minus lenses method

The subject viewed horizontal line of 2030 letters at 33cm as the examiner introduced minus

lenses in 025D increments until the target first becomes blurred According to the authors the

working distance adjustment used was kept at 250 DS to compensate for minification The

authors reported that monocular accommodative amplitude was significantly lower (p lt 0001)

in the group of poor readers (right eye 91 D 23 left eye 90 D 23) than in the control

group (right eye 105 D 17 left eye 105 D 17)

Helveston et al85 measured the amplitude of accommodation using the push up to blur

technique and reported that ldquoan ability to accommodate more than eleven dioptres was found in

between 90 and 93 of the studentsrdquo Ygge et al 102 assessed amplitude of accommodation

using the RAF rule and reported that the two groups performed similarly A similar technique

was used by Goulandris et al 90 and a lack of statistically significant difference between the

groups was reported

272 Accommodation Facility

In the same study Evans et al 100 reported that the dyslexic group appeared to be slower at a

62

test of accommodative facility The authors discredited the flipper technique as the

unnatural act of altering accommodation without changing vergence They further noted that

the flipper testhellip would still imperfectly reflect the normal visual environment owing to the

absence of a stimulus to vergence and accommodation

Grisham et al86 measured accommodative facility using the 200D lenses flipper technique

and the technique was performed as follows The 2D lenses was flipped each time the

student said the target was ldquoclearrdquo The target for this test was the 2030 target used for

accommodative amplitude The left eye was occluded The student was asked whether the

target was clear when held at about 40 cm If so then the student was told that the test would

be timed and that it should be reported whenever the target was ldquoclearrdquo The examiner then

started a stopwatch and counted the number of times the lenses were flipped in 30 seconds

That number was taken to represent the cycles per minute (cpm) and therefore to represent the

studentrsquos ability to change accommodation over time Based on the criteria used in their study

the authors reported that 76 of the students had adequate monocular accommodative facility

while 236 had ldquoweak accommodative facility of 9 cpm or less and 81 had accommodative

facility of as low as 1 or 0 cpm

In the study conducted by Alvarez et al 88 accommodative facility testing was conducted using

the following procedure The subject wearing his or her habitual correction was seated and

positioned so that the spectacle plane was 40cm from the Bernell Acuity Suppression Slide

(VO9) A 200 D accommodative demand was used with a 2030 test target at 40cm The

patient was asked to view the reduced Snellenrsquos letters 2030 in line 5 and say ldquonowrdquo when the

letters in that line appear clear and single and to report letters missing from lines 4 and 6

during the binocular testing They reported that the binocular accommodative facility values

were significantly lower (p lt 005) in the poor readers group (49 cpm 31) than the control

group (63 cpm 29)

63

Sucher and Stewart 98 assessed binocular accommodative facility at 35 cm using 150 diopter

flippers lens for ldquoone minuterdquo and ldquoused the University of California Berkeley clinic standard

of 20 cycles90s as a ldquocriterion for successrdquo found a higher prevalence of accommodative

infacility in dyslexics (222) than the control group (86)

On the contrary three authors6 12 93 found the control subjects to have better accommodation

facility than the dyslexic subjects The studies are Rosner and Rosner93 found a higher

prevalence of accommodative infacility among the control (10) than the learning disabled

group (9) Similarly Buzzelli6 found that the dyslexic subjects showed better

accommodation facility than the control group To assess accommodation facility ldquopatients

were asked to clear the number 9 on the Bernell Vectogramm Target at 40cm hellip alternately

viewed the target through a + 200DS and a ndash 200 DS lenshellip for 20 cycles) There was no

statistically significant relationship between the two groups

Keily et al12 assessed accommodative facility using +200DS and ndash 200DS flippers lenses

with an N6 fixation target for one minute at 30 cm rather than 40 cm because according to the

authors the children were required to hold the reading material They reported that

accommodative facility was better in normal readers than either of the poor reading groups

In another study Evans and Drasdo104 reviewed ophthalmic factors in dyslexia and commented

that convergence insufficiency accommodative dysfunctions and unstable eye movements are

more common in dyslexic population than in normal readers

Latvala3 simply noted that some aspects of accommodation were difficult to measure and failed

to give any details

64

273 Relative Accommodation

Alvarez and Puell88 measured relative accommodation using a phoropter The subject viewed a

horizontal line of 2030 letters at 40cm with hisher best refractive correction The examiner

introduced minus or plus spherical lenses in 025 D steps binocularly until the patient reported

first sustained blur Negative and positive relative accommodation values were similar in both

groups This was the only study accessed that reported on relative accommodation

274 Accommodation Posture

Evans et al100 assessed accommodation lag using the monocular estimate method of retinosocpy

and reported no statistically significant difference between the dyslexic and the control groups

Metsing and Ferreira103 measured the accuracy of the accommodative response (lag or lead)

objectively using the MEM retinoscopy in normal room illumination and found that the subjects

from the mainstream school demonstrated a high prevalence of lead of accommodation (40

and 405 for the right and left eyes respectively) However in the learning disabled group a

high prevalence of lag of accommodation (98 and 119 for the right and left eyes

respectively) was found The relationships were found to be statistically significant (p lt 005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000)

Sherman84 assessed accommodation amplitude and relative accommodation using minus lenses

and accommodative facility using the flipper techniques The lens powers for the plus and

minus lenses used to assess accommodation functions were not indicated His findings may be

difficult to utilize due to the somewhat unique way of classifying the results as ldquomechanical

problemsrdquo

Flax22 suggested further investigations in the nature of accommodative function in reading

65

disabled children indicating that the use of the push-up method of assessing the amplitude of

accommodation is unreliable due to its lack of sensitivity Flax22 reiterated that

accommodation has a contribution in learning disabilities and that accommodative function

inefficiencies tend to play an increasing role as the youngster moves through school and that

inefficient accommodation function is a significant contributor to lowered achievement in the

upper grade

28 Vergence Facility

Vergences are a clinical measure of an individualrsquos ability to overcome the prismatically

induced optical displacements of a target in space Convergence relates to each eye fixating

closer in space than the real physical location of the target while divergence is the measure of

the eyes to fixate further in space while still maintaining single binocular vision97

Buzzelli 6 assessed vergence facility as follows ldquopatients were asked to align a vertical picture

(box-X-O) slide made of anaglyph material for monocular presentation under binocular

conditions The target measured 21 mm in the vertical meridian and 10mm in the horizontal

meridian The subjects alternately viewed the target through 16 prism base out and 4 prism

base in The test was continued for 20 cyclesrdquo The author reported that the dyslexics

completed the vergence eye movement task significantly slower (300 seconds) than the normal

readers (240 seconds) A statistically significant relationship between the groups was reported

(p lt 005) It was recommended that the possible role of vergence in dyslexia should be

investigated on a larger sample of dyslexic and control groups

Metsing and Ferreira103 assessed vergence facility test at near (40 cm) with the subjects

presented with a vertical row of letters (69) using the 8 PD (BI) and 8 PD (BO) mounted in a

flipper device They reported that the prevalence of poor vergence facility (205) was found

to be higher in the mainstream group compared to the learning disabled group (183) The

relationship between the mainstream group and poor vergence facility (p = 0000) was found

66

to be statistically significant (plt005) A medium to strong relationship was found to exist

between the mainstream group and poor vergence facility (Cramerrsquos V = 0369) It was

concluded that children from mainstream schools are likely to present with poor vergence

facility compared to children from schools of the learning disabled

2 9 Fusional Reserves

The compensatory effect of fusional vergence reserves on phoria is very important as difficulty

with reading is likely to occur when there is uncompensated phoria in conjunction with

reduced fusional vergence reserves 92

A major study was reported by Evans et al 100 which found that negative and positive reserves

were significantly reduced in the dyslexic group relative to the control The ldquoprism vergence

amplitude calculated as the difference between base out and base in break and recovery points

was relatively reduced in the dyslexic grouprdquo They remarked however that several subjects

were unable to appreciate a subjective blur point but the results for the break and recovery

were complete

In the study conducted by Latvala et al 3 the incidence of fusional amplitude greater than or

equal to thirty two prism dioptre at near was higher in the dyslexic (75) than in the control

(61) group At distance they found fusional amplitude greater than or equal to15 prism

dioptre to be higher in the control (122) than in the dyslexic (94) group

Disordered vergence control was studied by Stein Riddell and Fowler 105 in the Uniteed

Kingdom in 1988 The subjects were thirty nine dyslexic children aged eight to eleven years

and a control group of twenty subjects of similar age group The vergence control was

measured using the Synoptophore They reported that two thirds of the dyslexic group had

67

abnormal vergence control According to the authors vergence eye movements never seem to

have been investigated in detail during reading before and that the study was the first in which

eye movement recording have been used to demonstrate differences between dyslexics and

normal readers

According to Grisham et al86 ldquoConvergence and divergence break and recoveryrdquo were

measured at 40 cm with a horizontal prism bar The student was first asked if the clown face

target on the smaller end of a standard optometric measure (Occlud-A-Measure Bernell

Mishawaka Indiana) could be detected If so then the student was requested to say whether

the target appeared single or double Once single vision was obtained the bar was moved

relative to the studentrsquos eye until double images were reported That point was considered

ldquobreakrdquo Then the motion of the bar was reversed until the student reported the image single

and this point was considered ldquorecoveryrdquo This procedure was first performed for base in (BI)

position of the bar over the left eye and repeated with base out (BO) position over the right

eye The authors reported that a large number of students had poor convergence skills 38

break at less than 18 PD and 95 recover at 7 PD or less These values were considered to be

in the ldquoweakrdquo to ldquovery weakrdquo range and remarked that in contrast to convergence lsquomodalrdquo

values for break (14 PD) and recovery (10 PD) were not in the ldquoadequaterdquo range but would be

classified as ldquovery weakrdquo for break and ldquoweakrdquo for recovery As a result very high

percentages of students had poor divergence skills 82 break at less than 20 PD and 60

recover at less than 11 PD

Kapoula et al 87 measured vergence reserves using prism bars at far and near distance for both

groups They reported that the median value for positive fusional vergence was 16 prism

diopters and 14 prism diopters for non-dyslexics and dyslexics respectively at far distance (p

= 012) and 20 prism diopters for both non-dyslexics and dyslexics at near distance For

negative fusional vergence they reported that ldquodivergence amplituderdquo was significantly

different in the two groups for both distances (p lt 0005) At far the median value was 6

68

prism diopters and 4prism diopters in non-dyslexics and dyslexics respectively and 12 prism

diopters and 10 prism diopters respectively at near

Bucci et al 89 measured fusional reserves at far and near using prism bars (base in and base

out) According to the authors orthoptic evaluation of vergence fusion capability showed a

significantly limited divergence capability for dyslexics compared to non-dyslexics the

median value was 10 prism diopters for dyslexics versus 17 prism diopters for non-dyslexics

In contrast convergence amplitude was in the normal range for the two populations while for

dyslexics the amplitude was significantly larger than for non-dyslexics the median value was

30 prism diopters and 18 prism diopters for dyslexic and non-dyslexic children respectively

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups

The mean fusional convergence capacities at distance were 1680 prism diopters for both the

control and the dyslexic group At near the corresponding figures were 2640 prism diopters

and 267 prism diopters respectively The mean fusional divergence capacity at distance was

650 and 620 prism dopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 105 and 102 prism diopters respectively There was no

statistically significant difference between the groups

210 Ocular Pathology

Ocular pathologies do not seem to be prevalent in the dyslexic population and it was not

reported on in the literature reviewed However according to Evans and Drasdo100 the

perspective to relate ocular pathology to dyslexia is in individuals who were normal readers but

developed reading disability due to pathological conditions such as stroke or tumors affecting

the right hemisphere These conditions may be relevant to the considerations of establishing the

relationship between ophthalmic problems and dyslexia100

69

Only one study83 included this as part of the investigation Hoffman83 reported that one child

(093) of the population of learning disabled children examined had acute conjunctivitis

A major study on dyslexic subjects (although outside the cope of the present study but worth

citing) was conducted by Raju11 in 1997 She studied oculomotor control in dyslexia and

reported that sixty five percent of the normal readers exhibited normal oculomotor control as

compared to the eleven percent of dyslexics and that fifty four percent of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills whilst twelve

percent of the control subjects displayed the same

70

LITERATURE REVIEW TABLE pages 70-72

71

72

73

211 Summary

Research into visual aspects of dyslexia has been undertaken mainly by optometrists and

ophthalmologists The main area of controversy is that ophthalmologists tend to deny any

relationship between vision and reading ability while optometrists assert that identifying and

correcting visual defects are an important part of managing dyslexia The findings of studies that

have investigated vision function in dyslexic population since the 1940s are inconclusive and

reflect the complexities associated with identifying the causes of dyslexic as well as its diagnosis

treatment and management

A range of study designs were used and the inclusion of a control group for example depended

on the objective of the study Some authors either compared the prevalence of vision defects in a

population of reading disabled group to the prevalence of similar vision defects in a group of

normal readers or only presented the prevalence of a particular visual condition in a group of

reading disabled children All the studies used convenience sampling methods at schools or by

including learners who were referred to their private practices the size being determined by the

number of dyslexic students available The main areas of vision functions studied were visual

acuity refraction binocular vision and ocular pathology The study findings were inconclusive

and the only vision variables consistently report ed across the studies was the association of

hyperopia with poor reading performance

The differences in results reported by the authors may be due to the methodological problems

such as differences in instrumentation and techniques failurecut-off criteria method of data

analysis lack of comparison group and subjectsrsquo selection from clinical population There was a

noticeable absence of studies on African children research has shown that African persons are

less at risk of myopia and more at risk for open-angle glaucoma than Caucasians There is

therefore scope for continued research among African children to determine whether there is any

relationship between dyslexia and vision

Research on dyslexia and vision that requires an interdisciplinary approach may yield different

results due to different approaches by different professionals However it is challenging to

74

design a study that would control for all variables possibly influencing reading ability and vision

function Reading performance is an extremely complex task and its measurement can be

influenced by many factors

Having reviewed the relevant literature on dyslexia and vision Chapter three will present the

research design the study population sampling procedure methods of data collection and

analysis

75

CHAPTER THREE METHODOLOGY

31 Introduction

This chapter will present the research design the study population sampling procedure

methods of data collection as well as method of data analysis

32 Research Design

The study was designed to provide empirical information to enable a comparison of the visual

characteristics of dyslexic children and normal readers A matched paired case-control

method was adopted Data was collected from both the dyslexic and control groups and

analysed

321 Sampling Design

Participants for both samples were selected using the convenient sampling method (based on

available subjects) and an optometric eye examination was conducted

322 Study Population and Participants

The study population comprised children attending a school for children with learning

difficulties from which the dyslexic children were selected to participate in the study while

the control group consisted of learners from a mainstream Durban school

i Dyslexic Group Participants for the study from the dyslexic group consisted of 31

African dyslexic children (the experimental group) selected from Khulangolwazi Special

School for children with learning difficulties in Clairwood South of Durban Psycho-

educational evaluationdiagnosis of dyslexia was not part of this study and was not

76

performed The school principal allowed access to all learnersrsquo file from which the

dyslexic learners where selected based on the school psychologistrsquos diagnosis Learners

attending this special school were referred from mainsream schools around Durban The

final diagnosis as to the type of learning difficulties was done by the school psychologist

None of the children were on any medication

The mean age for the dyslexic participants was 13 years and the learners were in grades

four through grade seven considering the fact that they were children that were reading two

grades behind their chronological age There were 15 boys and 16 girls Due to limited

number of learners who were classified as dyslexic it was difficult to recruit the targeted

number of one hundred participants for the study The subjects from both groups were

lsquoAfricanrsquo South Africans of African origin

ii Control Group Participants for the control group consisted of 31 children from a

mainstream school in Durban (Addington Primary School) The mean age of the children

was 11 years and 9 months A total of 15 boys and 16 girls were chosen ranging from grade

four to grade seven

323 Inclusion Criteria for the Dyslexic group

To be included in the study the subjects had to meet the following criteria

1 An average or above average intelligence quotient For this study an average was taken

as between 95 and above as recorded on the learners file by psychologist

2 Be two grades or more below grade equivalent in a mainstream school

3 The child has not been absent from school for more than 10 of the attendance days

The information on the learnersrsquo attendance was supplied by the school principal

324 Exclusion Criteria for the Dyslexic group

The following criteria resulted in children being excluded from the study

1 Presence of any emotional problems (information from the learnersrsquo file)

2 Presence of any systemic condition

77

3 Use of any systemic medication

The inclusion and exclusion criteria for the control group were similar to the experimental

group except that the children did not have any reading problems and were attending a

mainstream school

33 Ethical Clearance

The study protocol was approved by the then University of Durban-Westville research ethics

committee A letter of request for access to the schools was sent to the Department of

Education (Appendix B) Written informed consent (Appendix C) for access to the schools (for

both groups) was obtained from the Department of Education Written informed consents were

also obtained from the principals of the schools (mainstream and special school) (Appendix D)

Due to the difficulty in reaching the parents the school principals consented on behalf of the

parents for the learners to participate in the study The participants were fully informed of the

purpose of the study and accepted to co-operate with eye examination procedures

34 Testing Protocol

The testing was done first with the dyslexic learners and then at the mainstream school

35 Study Setting

The same testing arrangement was used for both groups A testing room was set up in one of

the offices provided by the school principal The distance acuity chart was placed at a point

six meters away from where the subjects sat and was used for the distance acuity chart and all

distance testing The examinations were done under room illumination except for some

procedures such as retinoscopy that required dim illumination resulting in the room being

dimmed

78

All testing was conducted in the morning as it was anticipated that better responses could be

obtained when the children were not tired Each school principal provided an assistant who

helped with various activities such as controlling room illumination The rationale and

technique for every procedure was fully explained to each participant and a trial reading was

taken to ensure that all instructions given were understood During the tests subjects were

asked (from time to time) if they were tired For any answer to the affirmative the testing was

discontinued and the child was allowed to have a break As a routine a break of 10 minutes

throughout the duration of entire procedure was allowed

The testing instruments were provided by the International Center for Eye Care Education

(ICEE) As the instruments were also being used by other researchers they were unavailable

on certain days resulting in some tests being postponed until a later date The data collection

took an average of two months per school All data were collected by the examiner alone and

each examination took an average of thirty minutes to complete

36 Data Collection Techniques

All tests were conducted in free space The data collection techniques used in the present

study was similar to techniques used in other studies 93100106-140 based on pediatric

populations (Table 31)

Table 31 Data Collection Tools and Measurement Outcome

InstrumentationTechnique Measurement Outcome

Bailey ndashLovie LogMAR Charts Visual Acuity

Royal Air Force (RAF) Rule NPC Amp of Accommodation

Welch Allyn Streak Retinoscope Objective Refraction

Trial Frame and Trial lenses Subjective Refraction

Cover Test - Occluder and Prism Bar Ocular Alignment

Maddox Rodpen torchPrism bar Heterophoria and Tropia

Maddox Wing Near Phoria

Prism Bar Heterophoria and Tropia

79

Pen torch PD rule Cornea reflex test-strabismus

Retinoscope and neutralizing lenses Accommodation Posture

Jackson cross cylinder Refine subjective findings

Trial Frame and neutralizing lenses Relative accommodation

+-2 flipper lenses Accommodation Facility

Direct Ophthalmoscope (Welch Allyn) Ocular Health

Prism Bars Fusional Reserves

361 Visual Acuity

Visual acuity is a measure of the acuteness or clearness of vision It assesses the ability of the

visual system to resolve detail which is dependent on the sharpness of the retinal focus within the

eye the sensitivity of the nervous elements and the interpretative faculty of the brain 106 Visual

acuity was assessed using the Logrithm of Minimun Angle of Resolution (LogMar) chart which

means that the letters change in size from line to line in equal steps of the log of the minimum

angle of resolution (MAR) Visual acuity was assessed monocularly and binocularly for both

distance and near The LogMar chart facilitates algebraic operations for statistical analysis and

allows for precise quantitative assessment of visual acuity It is recommended for use in research

studies in which visual acuity is a dependent variable106

Each subject was comfortably seated and the test was conducted with the examiner sitting in front

in such a way that the subjectrsquos view was not obstructed For monocular visual acuity the

subject was instructed to cover one eye with their right palm and to read the letters on the chart

This procedure was repeated binocularly and the test was done under normal room illumination 106 No subjects wore spectacles so visual acuity for aided vision was not indicated

362 Refractive Error

Refractive error was assessed objectively using the Streak Retinoscope (Welch Allyn) with a

+150D fogging lens (for an armrsquos length of approximately 67cm) with the subject fixating a

660 (to maintain fixation) optotype on the distance visual acuity chart As the subject focused

80

on the distance target on the chart the retinoscope light was passed across the subjects eye

(right hand used to scope right eye and left hand used to scope left eye) The motions

observed were neutralized with the appropriate lenses plus lenses for lsquowith motion and minus

lenses for against motion The subjective refraction was performed through monocular

fogging refined using the Jacksonrsquos cross cylinder followed by binocular balancing 106 The

retinoscopic results were used for analysis

Although retinoscopy was performed without cycloplegia as in other studies 395107108

cycloplegic refraction was not performed because the researcher (an optometrist) was not

licensed to use therapeutic drugs when the data was collected However it has to be

acknowledged that a non-cycloplegic examination among children has limitations among

those with hyperopia as the full extent of the hyperopia may be masked

363 Near Point of Convergence

The NPC was measured using the Royal Air force (RAF) rule The subject was instructed as

follows ldquoThis test measures your ability to turn your eyes in towards your nose Look

directly at the dot on middle of the line The image may appear to be blurring (not clear)

That is okay However if the dot becomes two say lsquotworsquo I will then pull the target away If

the dot becomes one again say lsquoonersquo rdquo The subject was asked to fixate on the dot on the

middle of the line as the target was advanced towards him The objective reading was taken at

the point when one eye loses fixation while the subjective reading was noted when the subject

reported the target to be double At a point when the subject reported double the target was

moved back until a point when the target was reported to be single again this was taken as the

recovery point The final break and recovery points recorded were the average of three tests

measurements The measurement was taken three times in order to detect fatigue which may

indicate poor convergence 111-113 The objective reading was taken for analysis

364 Strabismus

Strabismus was assessed using the Hirschberg test A penlight was held about 50cm from the

childs face in the mid plane and they were encouraged to fixate the light with both eyes open

81

The location of the corneal reflection on each cornea relative to the centre of the pupil was

then noted The displacement of the corneal reflection from the centre of the pupil (angle

lambda) was estimated in millimeters An ocular alignment was observed as a symmetric

displacement of the reflexes 114115 The examiner proceeded with the Kappa Test which is a

test of monocular fixation and is done while the child was monocularly viewing the examiners

penlight The penlight was maintained at 50 cm distance from the face The purpose of this

test was to determine the visual axis of each eye The examiner observed the location of the

corneal reflexes relative to the center of the pupil in the right eye and then in the left eye A

nasal displacement is signified by a (+) sign and a temporal displacement is signified by a (-)

sign with the amount quantified in millimeters away from center Again a normal location

will likely be either centered or +05 mm nasally displaced 114115

The interpretation of the magnitude and the type of deviation requires the data gathered from

both the Hirschberg tests The fixating eye is the eye in which the corneal reflex location is

the same on the Hirschberg and Kappa tests The strabismic eye is the eye in which there was

a difference The change in location from the Kappa tests as compared to the Hirschberg test

equals the amount and type of deviation Every 10 mm of corneal displacement is

approximately equivalent to 22 prism diopters A nasal change in location on Hirschberg

testing from the visual axis denoted by the Kappa test denotes an exotropia A temporal

change in location on Hirschberg testing from the visual axis denoted by the Kappa test

denotes an esotropia 114

365 Heterophoria

Ocular alignment was assessed using the cover test at six meters and 40cm for distance and

near respectively It is an objective technique and is very suitable for assessing ocular

alignment in children

For distance cover test the test target was a letter from the line above the subjectrsquos best visual

acuity of the worst eye The subject was advised that the muscle balance of their eyes was

assessed and that he should look steadily at the fixation target (an isolated letter on the chart)

at all times and ignore the occluder The coveruncover (unilateral) cover test was performed

82

to determine the presence of phoria or tropia 106114-115 To perform the unilateral cover test

the occluder was placed in front of the right eye held for approximately one to two seconds

and the left eye was observed for any movement If no tropia or phoria present the left eye

was covered and the right eye then observed Any movement observed was neutralized using

a loose prism bar in the corresponding base direction Base-in prism for exo deviation and

base-out for eso deviation

The alternating cover test was performed to assess the direction and magnitude of phoria or

tropia (it measures the total angle of deviation) 106114-115 The test conditions were the same as

the unilateral cover test The subject was comfortably seated and instructed to look at the

fixation target at distance The occluder was placed in front of the patientrsquos right eye held for

2-3 seconds and then quickly placed on the left eye The test was repeated several times with

the occluder held in front of each eye before quickly moving to the other eye while observing

the eye that was just uncovered Any deviation was neutralized accordingly 115 The Maddox

Wing was also used to assess near phoria under normal room illumination The technique was

assessed with the subject wearing his full subjective correction 115 The use of cover test for

the assessment of ocular alignment at both distance and near may have allowed for continuity

and standardization However the cover test and the Maddox wing were used by Evans et al 100 in the same study The subject was directed to look through the horizontal slits to view the

chart that comprised of a horizontal and vertical scale and a horizontal and vertical arrow The

construct of the Maddox wing is such that the right eye sees only the arrows while the left eye

sees only the scale It achieves dissociation by presenting independent objects to the patient

As the images are dissimilar the incentive to fusion is abolished and the eyes adopt the fusion

free position 115 The arrows are positioned at zero on the scales but through dissociation any

phoria will be indicated by an apparent movement of the arrow along the scale

To measure horizontal phoria the subject was asked ldquowhich white number does the white

arrow point tordquo The number on the scale represents the magnitude of the deviation and the

direction To measure vertical phoria the subject was asked to indicate which red number the

red arrow points to 115

83

366 Evaluation of Accommodation Functions

a Accommodation Posture The assessment of accommodation accuracy was performed at

the subjects habitual near distance using the monocular estimation method (MEM) The

MEM technique uses a specially made target with pictures or words suitable to the childs

reading level The card was attached to the retinoscope The procedure was performed

over subjects distance prescription and at his normal reading distance with enough light

to provide a comfortable reading vision Retinoscopy was performed on axis (right eye

before left eye) as the child reads the words or describes the picture on the card aloud As

the retinoscopic reflexes were observed an estimate of the magnitude of the motion was

made and trial lenses were then interposed briefly (approximately 05 seconds) to avoid

changing the childs accommodation status until no motion was observed The lens

power at which the motion was neutralized was taken as the lag or lead of

accommodation It is a lag of accommodation when the reflex is neutralized with

positive lenses while it is called lead of accommodation if neutralized with negative

lenses 112113116117

b Accommodation Facility this assesses the rate at which accommodation can be

stimulated and relaxed repeatedly during a specific time period It relates to the

individualrsquos ability to shift focus quickly and efficiently for varying distance The

procedure was performed with the best subjective correction in place monocularly and

binocularly but only the result for the binocular assessment was utilized for the data

analysis According to Wick and Hall 120 binocular accommodative facility testing may

not be a true accommodative facility measure such as monocular facility but clinically

binocular testing may be more useful because it gives a ldquoreal liferdquo assessment of

accommodation and binocular interactions of accommodation and vergence

The target used was letters on a 69 range on a near point card and at the subjectrsquos

habitual reading distance A lens flipper of 2 diopter lenses was used Letters and

pictures used were appropriate to the childs ability The subject was informed that he

would be shown two different lenses and that one pair causes the system to work while

84

the other pair relaxes it and was instructed to look at the materials as the lenses were

flipped Each time the print become clear and single he was to immediately report

clear The lenses were flipped and child should report clear when the material gets

clear To ensure that subject understood the instructions trial readings were first taken

The number of cycles completed per minute was recorded One cycle meaning clearing

both the plus and minus lens sides The test was done binocularly112-113118-120 At near

with no suppression control

c Amplitude of Accommodation The amplitude of accommodation was assessed by

Donderrsquos push ndash up method using a Royal Airforce (RAF) near point rule a rod with a

movable target and metrics as well as dioptric marking This procedure was performed

monocular and binocularly The subject was seated comfortably and instructed to read a

line of letters on the card that corresponded to a visual acuity of 69 and told to keep the

letters clear The target was slowly moved towards the child along the rule until the child

reported first sustained blur at which point the dioptric result was read off the Royal air

force rule Three readings were performed and an average taken This test was

conducted monocularly for each eye and then binocularly

As reported by Wick and Hall 120 the minimum amplitude of accommodation expected is

based on the subjects age and can be estimated from the Hofsetterrsquos formula

Minimum =15 - 025 (age)

Expected =185 - 03 (age)

Maximum = 25 - 04 (age)

The amplitude of accommodation was generally considered clinically important only

when it falls below the expected age minimum 120

d Relative Accommodation (RA) Relative accommodation tests are plus-to-blur (negative

relative accommodation) (NRA) and the minus-to-blur (positive relative accommodation)

(PRA) The relative accommodation tests assess patientsrsquo ability to increase and decrease

accommodation under binocular conditions when the total convergence demand is constant 106 It is also an indirect assessment of the vergence system since the vergence demand

85

remains constant while accommodative demand varies 121122

Relative accommodation was measured using the plus and minus lenses binocularly at

near with distance subjective results in place 122 In this procedure the child viewed a

reading target size letter size or picture of 66 size at the reading distance with the best

subjective correction in place Plus or minus lense powers was added in 025 diopter

steps in approximately 2 seconds interval121 to assess negative and positive

accommodation respectively This test was performed binocularly and the reading was

taken when the subject reported a sustained blur The relative accommodation is difficult

to assess with trial lenses121 The trial lenses were used to assess relative accommodation

in the present study The use of the phoropter was not possible considering the study

setting

367 Fusional Reserves

The term fusional reserve is used synonymously with vergence reserves and prism vergence

vergence amplitude 121123 or fusional amplitudes 111 Vergence ability is measured using

prisms placed in front of each eye The amount of prism is increased gradually to measure the

amount of fusional reserve the individual has in reserve to compensate for a phoria112 124 The

amount of base out prism required to produce diplopia is called positive fusional reserves or

positive fusional vergence while the amount of base in prism required to produce diplopia is

called the negative fusional reserves or negative fusional vergence 115

Positive fusional vergence (convergence) and negative fusional vergence (divergence) were

assessed using a prism bar in steps without suppression control although it has been reported

by Wesson et al 125 that when suppression is controlled the average vergence values will be

lower because the test is stopped when the suppression is detected that is if suppression is not

monitored the break is not detected until the stimulus is outside the suppression zone and a

higher vergence value may be obtained However

Scheiman et al 126 argued that the significance of such suppression in binocular individuals is

unknown

86

Assessing fusional reserves in children with loose prism bars may be a better technique

because it offers the possibility of viewing the eye movement objectively Secondly it

removes the restriction imposed by testing children behind the phoropter Finally the test

itself nearly duplicates the movement of the eyes under normal conditions allowing

convergence or divergence in discrete steps rather than as a smooth-pursuit type vergence

movement 125

The test was performed with the target at six meters (test target 69 letter line) and at 40 cm

(N5 text) The prism was placed in front of the right eye first Base-in was routinely assessed

first the rational for this sequence of testing being that the convergence response stimulated

during the base-out (or convergence) measurements may produce vergence adaptation ( a

fusional after-effect) which may temporarily bias the subsequent base in values in the base-out

direction 127 Typically there is no blur point for base -in vergence testing at distance This is

because at distance accommodation is already at a minimum and cannot relax beyond this

point 115

Base out prisms were used to measure positive fusional vergence while base in prisms were

used to measure negative vergence The subject was seated comfortably and instructed as

follows ldquoI am measuring the ability of your ayes to converge and diverge Watch the row of

letters Tell me when they first get blurry if they do Also tell me when they first become

two rows of letters and when they become one againPrime The prism was introduced over the

right eye lsquoDoublersquo was first demonstrated to the child by placing a 20 diopter prism over one

eye If blur was reported the prism power was increased until a break was reported The

prism power was then reduced until the subject reported that the rows of letters were single

The technique was then repeated for base out prism and then the test was repeated at near The

break and recovery points were determined subjectively from the childrsquos report of blur break

and recovery and objectively by observing the subjects eye movements The objective

findings were used for analysis

87

368 Ocular Health

The only test performed to evaluate ocular health was direct ophthalmoscopy

369 Vergence Facility

It was impossible to perform the vergence facility testing as the children could not fuse the

base in prisms So vergence facility data was not available for analysis The recovery point

of the base in reserves may provide useful information on the vergence facility According to

Bishop 111 ldquoblur and break points tend to reflect the quantity of fusion whereas the recovery

point indicates the quality of fusion that is the ease of change of fusional demands (facility)

and the ability to maintain fusion (stamina)

37 Statistical Analysis

The data entry was done by a staff optometrist who is skilled in statistical analysis The data

analysis was undertaken by the International Center for Eye Care Education statistician Data

was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 For comparison all data from both groups was subjected to a two-sample t-test (2-

tailed) (Table 32) Participants who did not meet the pass criteria for any of the test variables

were referred to their optometrist for further evaluation

Table 32 The Diagnostic Criteria for Each Test Variable

Variable Diagnostic Criteria

Visual Acuity 128-131 69

88

Refractive Errors 128-131

Hyperopia ge +075

Myopia ge -050

Astigmatism ge -075

Anisometropia ge 075 between the two eyes

Emmetropia lt - 050DS lt +100DS lt -075DCyl

Near Point of Convergence 132-134 ge 10cm

Accuracy of Accommodation 116117119120

Lag ge +075 Lead any minus finding

Accommodative Infacility 118119 gt 7cycles per minute in response to +2 -200 flipper testing

binocularly at near

Amplitude of Accommodation106119120

Binocular accommodative amplitude ge 2D below the expected

value for the patients age for minimum amplitude (using

Hofsetterrsquos formula) (15-14age)

Positive Relative Accommodation 106121122124

gt - 237DS

Negative Relative Accommodation 106121122124

gt + 2 DS

Phoria 106128

Distance

Near

gt 6 prism diopters exophoria or 4 prism diopters esophoria

gt 6 prism diopters exophoria or 4 prism diopters esophoria

Strabismus 131135 Manifest or intermittent manifest deviation of gt 2 prism

dioptres 135 or asymmetry in Purkinje reflex or compensating

fixation a misalignment of ge 25 degrees 131

Fusional Reserves LimitsDistance 119

Near 106121126

Base in Blur X Break 6 Recovery 4

Base out Blur 10 Break 16 Recovery 10

Base in Blur 14 4 Break12 5 Recovery 7 4

Base out Blur 22 8 Break 23 8 Recovery 16 6

89

38 SUMMARY

The study consisted of two groups of 31 participants from two schools in Durban The study

group was children with learning difficulties from which the dyslexic children were selected

and the control group was selected from a mainstream school The mean age for the dyslexic

participants were 13 years while the mean age of the children from the control group was 11

years and 9 months A convenience sampling method was adopted to select the study

participants due to the limited number of subjects to select from

The vision functions investigated may be divided into three broad areas of visual acuity and

refraction binocular vision and ocular pathology which corresponds with areas of vision

function investigated in the literature review Standard optometric testing procedures were

used to assess the vision variables

Visual acuity was assessed using the LogMAR chart at both distance and near Refractive

error was determined objectively using a streak retinoscope (without cycloplegia) while the

child fixated a 660 optotype at 6 meters with a fogging lens to control accommodation

The near point of convergence was measured using the Royal Air force rule (RAF) Ocular

alignment was assessed using the cover test at six meters and 40cm for distance and near

respectively For distance cover test the test target was a letter from the line above the

subjectrsquos best visual acuity of the worse eye The coveruncover (unilateral) cover test was

performed to determine the presence of a phoria or tropia The alternating cover test was

performed to assess the direction and magnitude of the phoria or tropia The test conditions

were the same as the unilateral cover test The Maddox Wing was also used to assess near

phoria under normal room illumination Strabismus was assessed using the Hirschberg test

with a penlight held about 50cm from the childs face in the mid plane and was encouraged to

fixate the light with both eyes open The location of the corneal reflection on each cornea

relative to the centre of the pupil was then noted

The different aspects of accommodation functions were also measured The assessment of

accommodation accuracy was performed at the subjects habitual near distance using the

90

monocular estimation method (MEM) The accommodation facility was assessed using a 2

diopter lens flipper with the best subjective correction in place The target used was letters on

a 69 range on a near point card and at the subjectrsquos habitual reading distance The amplitude

of accommodation was assessed by Donderrsquos push ndash up method using a Royal Airforce (RAF)

near point rule This procedure was performed monocular and binocularly with a 69 visual

acuity as the test target The relative accommodation was measured using the plus and minus

lenses The test target is a reading letter target size or picture of 66 size at the reading distance

with best the subjective correction in place This test was performed binocularly and the

reading was taken when the subject reported a sustained blur

The fusional reserves were assessed using prism bars in steps without suppression control

The test was performed at 6 meters (test target 69 letter line) and at 40 cm (N5 text) Base out

prisms were used to measure positive fusional vergence while base in prisms were used to

measure negative vergence Ocular health evaluation was assessed using the direct

ophthalmoscope

Data was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 To compare the data from both groups all data was subjected to a two sample t-test

(2-tailed) The methods used in this study were the same as those used for similar tests in the

international literature reviewed in Chapter Two and the sample size was larger than most of

the studies presented

Chapter Four will outline the study results with the comparative and descriptive statistical

analysis

91

CHAPTER FOUR RESULTS

41 Introduction

This chapter presents the study findings and the analysis of results obtained The data for the

prevalence of visual acuity refractive errors near point of convergence accommodation

functions and heterophoria are presented in histogram while the descriptive statistics are

presented in tables

42 Prevalence of Vision Defects

The prevalence is an indication of the subjects that have either achieved or did not achieve the

pass fail criteria and is not a normal distribution of a particular condition The prevalence rate

of the vision functions is expressed in percentages The other data is expressed as mean (M)

and standard deviation (SD) and calculated for descriptive purposes at 95 confidence

interval (CI)

If an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants would be expected

421 Visual Acuity (VA)

i Dyslexic Group In total 32 of subjects in the dyslexic group had visual acuity

less than 69 while 68 had visual acuity of 66 and 65 The distribution of those

subjects with VA le 69 is as follows 65 (2 subjects) had VAs of 69 65 (2

subjects) had VA of 612 65 (2 subjects) had VA of 615 3 (1 subject) had

VA of 630 3 (1 subject) each had VA of 638 648 660 respectively (Fig

41)

ii Control Group In total 32 of subjects in the control group had visual acuity

92

less than 69 while 68 had visual acuity of 66 The distribution is as follows 3

(1 subject) each had VA of 69 612 615 624 while 65 (2 subjects) had VA

of 630 638 and 648 respectively (Fig 41)

Fig 41 Prevalence of Visual Acuity

The mean visual acuity for the right eye for the dyslexic group was 017 031 and 000

024 for the right eye for the control group There was no statistically significant difference

between the two groups (p = 029) (Table 41) The mean visual acuity for the left eye for the

dyslexic group was 020 033 and 000 024 for the control group There was no

statistically significant difference between the groups (p = 023) (Table 41) LogMAR acuity

of 020 is equivalent to 69 in Snellenrsquos notation (Appendix E Visual acuity conversion table)

Table 41 Descriptive Statistics for Visual Acuity Variable Group N ( 95 CI) Minimum Maximum p

Mean (LogMAR)

SD

VAcuityRE

LE

DyslexicControl

DyslexicControl

3131

3131

017000

020000

031024

033024

000000

000000

010090

100090

029

023

422 Refractive Error

i Dyslexic Group Twenty three percent (23) of the dyslexic subjects had

refractive errors while seventy seven percent (77) were emmetropic (defined as

93

retinosocpic findings of less than 05 myopia less than +075 hyperopia or less than

075 astigmatism) (Fig 42) The distribution is as follows 65 (2 subjects) had

myopia 65 (2 subjects) had hyperopia (both latent) As cycloplegia was not

used in assessing refractive error an estimate of the diagnosis of latent hyperopia

was made based on a high difference between the retinoscopic findings and the

subjective refraction results In the event that the increased retinoscopic finding

(that is more plus) did not blur the subjectsrsquo vision latent hyperopia was assumed

About 10 (3 subjects) had hyperopic astigmatism 65 (2 subjects) had

amblyopia (defined as visual acuity in either eye of 2040 or a two-line difference

in acuity between the two eyes with no improvement with pinhole) 141 and 65 (2

subjects) had anisometropia (defined as a difference of ge 075 in sphere or cylinder

between the two eyes) (Fig 43)

ii Control Group Twenty two and a half percent (225) of the participants from

the control group had refractive errors classified as follows 65 (2 subjects)

had myopia 3 (1 subject) had hyperopia and 13 (4 subjects) had astigmatism

and 65 (2 subjects) had anisometropia No participant had amblyopia

Fig 42 Prevalence of Refractive Errors (total

01020304050607080

Myopia HyperopiaAstimatism (Hyperopic)Anisometropia Amblyopia Emmetropia

Prevalence

Dyslexic GroupControl Group

0

5

10

15

20

25

Prevalence

Refractive Errors (total)

Dyslexic GroupControl Group

94

Fig 43 Prevalence of the Types of Refractive Errors

Refraction data for two children from the dyslexic group who had cataracts could not be

obtained due to poor reflexes and were excluded from the analysis

The mean spherical equivalent (SE) refraction for the right eye for the dyslexic group was 086

098 and 070 103 for the control group was There was no statistically significant

differences between the two groups (p=066) The mean spherical equivalent refraction of the

left eye for the dyslexic group was 057 101 and 049 109 for the control group There

was no statistically significant difference between the two groups (p= 092) (Table 42)

Table 42 Descriptive Statistics for Refractive Error Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

RE (SE)

LE (SE)

DyslexicControl

DyslexicControl

2931

2931

086070

057049

098103

101109

-125-350

-150-350

500350

400350

066

092

423 Near Point of Convergence

i Dyslexic Group Thirty three percent (33 10 subjects) had NPC break points

(objective) of greater than or equal to 10cm while about sixty seven percent 67

(20 subjects) had NPCs of less than 10cm (Fig 44) One child complained of being

tired and did not participate in this procedure and was therefore excluded

ii Control Group Forty eight percent (48 15 subjects) had NPC break point

(objective) of greater than or equal to 10cm Fifty two percent (16 subjects) had

95

NPC less than 10cm (Fig 44) Data on NPC for one child from the dyslexic group

was lost

The mean NPC break was 890 503 for the dyslexic group and 1260 870 for the control

There was a statistically significant difference between the two groups (p = 0049) The mean

NPC recovery was 1400 588 for the dyslexics and 2200 820 for the control group

There was a statistically significant difference between the two groups (p = 006) (Table 43)

Table 43 Descriptive Statistics for NPC Break and Recovery Points Variable Group N (95 CI) Minimum Maximum P

Mean SD

NPC Break

NPC Recovery

DyslexicControl

DyslexicControl

3030

3030

8901260

14002200

503870

588820

500400

600800

26003400

28003800

049

006

424 Ocular Health

Six and a half percent (2 subjects) in the dyslexic group had cataracts (unclassified) on both

eyes No pathology was detected in the control group

425 Heterophoria

i Dyslexic Group No subject in the dyslexic group manifested with a phoria at

96

distance

At near 3 (1 subject) had an esophoria greater than or equal to 4 prism diopters

3 (1 subject) had an esophoria of 3 prism diopters 65 (2 subjects) had

exophorias of greater than or equal to 4 prism diopters 3 (1 subject) had an

exophoria greater than or equal to 8 prism dioptres and 65 (2 subjects) had

exophorias of 6 prism diopters (Fig 45)

ii Control Group At both distance and near no subject in the control group had

phoria of greater than 2 prism diopters

Fig 45 Prevalence of Phoria at Near (40cm) with Maddox Wing

Four children from each group could not complete the test as they left to attend lessons The

mean exophoria for the dyslexic group was 163 261 and 180 042 for the control group

There was no statistically significant difference between the two groups (p = 059) The

mean esophoria for the dyslexic group was 350 070 and 2 000 for the control group

There was no statistically significant difference between the two groups (p= 046) (Table 44)

No tropias were observed

Table 44 Descriptive Statistics for HeterophoriaVariable Group N ( 95 CI) Min Maxi P

97

Mean SD

At NearExophoria

Esophoria

DyslexicControl

DyslexicControl

2727

2727

163180

350200

261042

070000

000100

300200

1000200

400200

059

0469

426 Accommodation Functions

a Accommodation Posture As shown in Figure 46 the distribution of accommodation

lag is as follows

i Dyslexic Group

Thirty nine percent (11 subjects) had a lag of accommodation (defined as + 075 or

greater) three and a half percent (1 subject) had lead of accommodation (defined as

- 025 or greater) while fifty seven percent had normal accommodation posture

ii Control Group

Forty two percent (13 subjects) had lag of accommodation About eleven percent

(11) (3 subjects) had lead of accommodation while about thirty nine percent had

normal accommodation posture

Three children from the dyslexic group could not continue with this test as they had to

leave to attend class activities while three children from the control group sought

permission to be out of the testing room at that point These children were excluded

from the analysis for binocular accommodation lag

98

Fig 46 Prevalence of Accommodation Lag ( ge+075)

The mean accommodation lag for the right eye was 091 038 for the dyslexic group

and 092 057 for the right eye for the control group There was no statistically

significant differences between the two groups (p =083) The mean for the left eye for

the dyslexic group was 085 036 and 091 048 for the control group There was no

statistically significant difference between the two groups (p = 061) (Table 45)

Table 45 Descriptive Statistics for Accommodative PostureVariable Group N (95 CI) Minimum Maximum P

Mean SD

Accomm lag

RE

LE

DyslexicControl

Dyslexic Control

2828

2828

091092

085091

038057

036048

000-050

-050-050

200200

125200

083

061

b Accommodative Facility As shown in Figure 47 the distribution of accommodation

facility was as follows

i Dyslexic Group

Forty six percent (46 13 subjects) had normal accommodative facility (defined as

0

10

20

30

40

50

Prevalence

Bi no cu lar A ccom m od ation Lag

D y s lex ic G r o u p

C on t ro l G ro u p

99

greater than 7cpm) binocularly at near while (54) (15 subjects) had

accommodation facility less than or equal to 7cpm (Fig 47)

ii Control Group

Sixty seven percent (67) (18 subjects) had normal accommodative facility

(defined as greater than 7cpm) while thirty three percent (33) (9 subjects) had

accommodation facility less than 7cpm

Three children from the dyslexic group and four from the control did not complete the test

as they indicated that they were tired The mean binocular accommodation facility was

686 cpm 274 for the dyslexic group and 885 369 for the control group There was a

statistically significant difference between the two groups (p=0027) (Table 46)

Table 46 Descriptive Statistics for Binocular Accommodative FacilityVariable Group N ( 95 CI ) Minimum Maximum p

Mean SDAccommodativeFacility at near

DyslexicBE

ControlBE

28

27

686

885

274

369

2

2

12

21

002

7

c Amplitude of Accommodation Monocular amplitude of accommodation in both eyes

and for both groups did not differ significantly (not more than 050D) except for one

subject who had a difference of 4 diopters between the two eyes The amplitude of

100

accommodation for two subjects who had latent hyperopia in the dyslexic group was

relatively low (600DS and 800DS) as compared to age minimum amplitude of

accommodation of 1175DS The amplitude of accommodation for two participants who

had cataracts were excluded from the analysis

Only the data for the monocular amplitude was analyzed When accommodation

amplitude is assessed monocularly it measures the response for each eye individually

and is particularly important to determine whether a patient has accommodative

insufficiency 133 The monocular amplitude of accommodation has been used to assess

amplitude of accommodation function in several studies 86 88100102-103 The mean for the

right eye for the dyslexic group was 1198 234 and 1287 108 for the control group

There was no statistically significant difference between the two groups (p =007) (Table

47) The mean for the left eye for the dyslexic group was 1214 214 and 1287 116

for the control group There was no statistically significant difference between the two

groups (p =022) (Table 47)

Table 47 Descriptive Statistics for Amplitude of Accommodation Variable Eye Group N ( 95 CI ) Minimum Maximum p

Mean SDAmplit of Acomm

RE

LE

DyslexicControl

DyslexicControl

2931

2931

11981287

12141287

234108

215116

810

810

2015

2015

007

022

d Relative Accommodation (NRA PRA) As shown in Fig 48 the results for the relative

accommodation were as follows

For the PRA all subjects in both groups had PRAs of more than -200DS For the

negative relative accommodation (NRA) 89 of the dyslexic subjects had NRA greater

than +200DS while 96 of subjects from the control group had NRA greater than

+200DS

101

Fig 48 Prevalence of Relative Accommodation (NRA+2PRA-200)

Three children from the dyslexic group and five from the control group could not

continue with the test for the assessment of relative accommodation as they had to attend

important class activities

The mean PRA for the dyslexic group was -623 117 and -606 063 for the control

group There was no statistically significant difference between the two groups (p

=051) The mean NRA for the dyslexic group was 322 079 and 311 047 for the

control group There was no statistically significant difference between the two groups

(p =068)(Table 48) The findings for RA were higher than the published norm This

may be due to the process of addition of trial lenses

Table 48 Descriptive Statistics for Relative Accommodation Variable Group N ( 95 CI ) Minimum Maximum p

Mean SDPRA

N RA

DyslexicControl

DyslexicControl

2826

2826

-623-606

322311

117063

079047

-900-700

200200

-400-500

60045

051

068

427 Fusional Reserves

The children in both groups either could not report or understand blur so the result for break

102

and recovery was used in all analysis of vergence function

a Base-in Vergence Reserves at Distance two children from the dyslexic group could not

complete all aspects of the fusional reserves assessment because they were tired These

children were excluded in the analysis

The mean base-in to break for the dyslexic group was 1469 683 and 1600 350 for

the control group There was no statistically significant difference between the two

groups (p =046) The mean base in to recovery for the dyslexics was 1172 620 and

1280 317 for the control group There was no statistically significant difference

between the two groups (p =049) (Table 49)

Table 49 Descriptive Statistics for Base-in Vergence Reserves at distanceVariable Group N (95 CI) Minimum Maximum P

Mean SD

BI to Break (dist)

BI to Recovery (distance)

DyslexicControl

DyslexicsControl

2931

2931

14691600

11721280

683350

620317

410

28

4022

3520

046

049

b Base-in Vergence Reserve at Near The mean base in to break at near was 1185 514

for the dyslexics and 1283 313 for the control group There was no statistically

significant difference between the two groups (p=029) The mean base in to recovery was

872 478 for the dyslexics and 1032 335 for the control group There were no

statistically significant differences between the two groups (p=017) (Table 410)

Table 410 Descriptive Statistics for Base-in Vergence Reserves for NearVariable Group N (95 CI) Minimum Maximum P

Mean SDBI to Break (near)

BI to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

11851283

8721032

514313

478335

26

14

2518

2015

029

017

103

c Base-out (BO) Vergence Reserves at Distance Four children from the control

group could not give a report on the recovery point Two children from the dyslexic group

could not complete the test The mean base out to break at distance for the dyslexic group

was 2706 925 and 2416 975 for the control group There was no statistically

significant difference between the two groups (p=024) The mean BO to recovery for the

dyslexic group was 1876 796 and 17 693 for the control group There was no

statistically significant difference between the two groups (p= 040) (Table 411)

Table 411 Descriptive Statistics for Base-out Vergence Reserves at Distance Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

BO to Break(Distance)

BO to Recovery(dist)

DyslexicControl

DyslexicsControl

2931

2927

27062416

18761700

925975

796693

10001000

400600

0004000

35003500

024

040

d Base-out Vergence Reserves at Near The mean base-out to break at near was 2160

1162 for the dyslexic group and 2109 842 for the control group There was no

statistically significant difference between the two groups (p =084) The mean base-out

to recovery at near for the dyslexic group was 1335 745 and 1555 625 for the

control group There was no statistically significant difference between the two groups

(p =016) (Table 412) The comparative and descriptive statistsics for all variables is

shown in Table 413

Table 412 Descriptive statistics for base-out vergences at near Variable Group N (95 CI) Minimum Maximum P

Mean SD

BO to Break (near)

BO to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

21602109

13351555

1162842

745625

8001000

600800

4040

3530

084

016

104

Table 413 Comparative Descriptive Statistics for Dyslexic and Control groups

Variable Dyslexics( 95 CI)

Control( 95 CI)

N Mean SD N Mean SD PVisual Acuity

RE 31 017 031 31 000 024 029LE 31 020 033 31 000 024 023

Refractive ErrorRE 29 086 098 31 070 103 066LE 29 057 101 31 049 109 009

NPC Break 30 890 503 31 1260 870 049NPC Recovery 30 14 588 31 2200 820 006

Heterophoria (Near)Exo 27 163 261 27 180 042 059Eso 27 350 070 27 200 000 046

PRA 28 -623 117 26 -606 063 051NRA 28 322 079 26 311 047 068

Amp of AccommodationRE 29 1198 234 31 1287 108 007LE 29 1214 215 31 1287 116 022

Accom Facility (bin) 28 686 274 27 885 369 003

Accomm LagRE 28 091 038 31 092 057 083LE 28 085 036 31 091 048 061

BI Break (Dist) 29 1469 683 31 1600 350 046BI Recovery (Dist) 29 1172 620 31 1280 317 049

BI Break (Near) 29 1185 514 31 1283 313 029BI Recovery (Near) 29 872 478 31 1032 335 017

BO Break (Distance) 29 2706 925 31 2416 975 024BO Recovery (Distance) 29 1876 796 31 1700 693 040

BO Break (Near) 29 2160 1162 31 2109 842 084BO Recovery (Near) 29 1335 7 45 31 1555 625 016

105

Summary Statistical tests were performed on the data to ensure scientific validity and were presented for

comparative and descriptive purposes The prevalence of visual acuity was similar between the

dyslexic and control groups and there was no statistically significant difference between the two

groups (p = 029 right eye 023 left eye)

The prevalence of total refractive errors was similar between the two groups and there was no

statistically significant difference between the two groups (p = 066 right eye and 092 left

eye) Hyperopia was slightly more prevalent in the dyslexic group than in the control group

The prevalence of myopia was the same in both groups Astigmatism was more prevalent in

the control group than in the dyslexic group The prevalence of anisometropia was the same in

both groups Amblyopia was more prevalent in the dyslexic group compared to the control

group

The control group had a higher near point of convergence break and recovery points than the

dyslexic group The NPC showed a statistically significant difference between the two

groups NPC break (p =0049) and NPC recovery (p =0006) The prevalence of exophoria at

near was higher in the dyslexic than the control group There was no statistically significant

difference between the two groups (p= 059) The prevalence of esophoria in the dyslexic

group was higher than that found in the control group but there was no statistically significant

difference (p =046)

The prevalence of relative accommodation was similar in both groups and there was no

statistically significant difference between the groups The dyslexic group had a reduced

amplitude of accommodation compared to the control group and there was no statistically

significant difference between the two groups (p=007 right eye andp = 022 left eye) The

dyslexic subjects performed significantly worse than the control subjects in accommodative

facility function and there was a significant difference between the two groups (p =0027)

The dyslexic subjects had a lower accommodation lag than the subjects from the control

group There were no statistically significant differences between the two groups right eye (p

= 083) and left eye (p = 060)

At near the negative fusional vergence (base in vergences)) was reduced in the dyslexics

compared to the control group while the positive fusional vergence (base out vergences) was

similar for both groups There was no statistically significant difference between the groups

106

(p = 021 for BI break at near) and (p = 084 for BO break at near) Having analyzed the

results Chapter Five will discuss the findings in the context of the international studies

reviewed in the literature

107

CHAPTER FIVE DISCUSSION

51 Introduction

A review of the literature reveals that studies 2356878990 conducted on dyslexic children have

investigated the prevalence of vision defects in the dyslexic population and examined whether

such vision anomalies are correlated with dyslexia by comparing vision characteristics in

dyslexic subjects to normal readers

The aim of this study was to determine the prevalence of vision defects in a South African

population of the dyslexic school children and investigate the relationship between vision and

dyslexia by comparing the vision characteristics of the dyslexic to the control group of learners

from a mainstream school The proposed null hypothesis was that there was no statistically

significant difference between the means of the vision functions in the dyslexic population

compared to the control group A discussion of the results of this study is presented

52 Prevalence of Vision Defects

521 Visual Acuity

The 32 prevalence of visual acuity worse than 69 was the same for both groups A reduced

visual acuity could be due to ocular abnormality (such as cataract) or due to normal refractive

error differences in the population Visual acuity defects due to ocular diseases are usually not

common in paediatric populations142 but tend to be related to refractive error changes in the

population143 This relates more in the present study as the prevalence of defects of visual acuity

and the total refractive errors were similar Consequently in relating visual acuity to refractive

errors myopia astigmatism and hyperopia may reduce distance or near visual acuity

108

Visual acuity testing is widely performed in screening refraction and monitoring of disease

progression It is also used in both basic and clinical vision research as a way to characterize

participants visual resolving capacities144 Specific to the present study visual acuity testing

helps detect visual problems at the distances at which most school learning activities occur and

gives useful preliminary information on how good or poor a childrsquos vision is reduced distance or

near visual acuity gives an indication of poor vision that will necessitate further examination

However in assessing how visual acuity relates to reading performance it is important to note

that visual acuity as measured with the traditional letter identification only assesses the ability to

discern letters on the visual acuity charts and does not assess dynamic vision behaviour

binocular eye control or ocular stress and so may not necessarily mean normal vision

Furthermore normal visual acuity may not necessarily mean normal vision since some people

may have other vision defects such as color vision or reduced contrast or inability to track fast-

moving objects and still have normal visual acuity The reason visual acuity is very widely used

is that it is a test that corresponds very well with the normal daily activities a person can handle

and evaluates their impairment to do them 145

In relation to previous studies the visual acuity findings in this study are similar to reports by

Buzzelli 6 Goulandris et al 90 and Metsing and Ferreira 103 On the contrary Hoffman 83 found

a lower prevalence (1028) of visual acuity worse than 69 in a group of learning disabled

children In contrast to the present study however the mean age of subjects in Hoffmanrsquos study

was 8 years 6 months while the mean age in the present study was 13 for dyslexics and 11years 9

months for the control group Grnlund et al 140 documented that age must be taken into

consideration when describing and comparing VA in different populationsbecause VA develops

from birth to adolescence A lower prevalence (10) of visual acuity worse than 69 was also

reported by Sherman84 Latvala et al 3 reported a slightly worse distance acuity in the dyslexic

group than in the control group but noted no statistically significant difference Evans et al 5

found that dyslexic groups had a significantly worse visual acuity than the control group

(Binocular distance VA p =00164 and binocular near VA p =00018) Similarly Ygge et al2

reported that the subjects from the control group had a better visual acuity than the dyslexic

group at both distance and near and that the results showed statistically significant differences

(distance at p =003) (near at p=0005) Bucci et al 89 and Kapoula et al 87 reported the

109

prevalence of visual acuity of all participants in their study to be normal A study conducted by

Grisham et al 86 lacked a control group but it was reported that 568 of the participants had 66

acuity in both eyes (compared to 68 in dyslexic group in the present study) 172 had 69 or

worse (compared to a total of 315 who had visual acuity of 69 and worse in the present study)

The mean age of the participants in the study by Grisham et al 86 was 154 years while the mean

age of the participants in the dyslexic group in the present study was 13 years

The factors which may affect the outcome of visual acuity measurements that may lead to

variations in results reported by different authors include size position and distribution of retinal

mosaics (retinal photoreceptors) 143 optical aberration pupil size clarity of the optical media

age method and type of chart used contrast isolated or multiple letters state of accommodation

illumination as well as the criteria used to define visual acuity114 140 Psychological factors

affecting visual acuity testing results include blur interpretation fatigue and malingering 143

These factors may be difficult to control and could constitute confounding variables which may

lead to variations in visual acuity results reported by different authors

522 Refractive Error

The prevalence of uncorrected refractive error of 23 found in the dyslexic group was similar to

the 225 found in the control group and there was no statistically significant difference between

both groups (p = 066 right eye p = 092 left eye) This finding may mean that the dyslexic

subjects are not at more risk of a particular refractive anomaly compared to participants from the

control group as the dyslelxic gourp has similar distribution of refractive errors

Some studies 5838498103146 did not classify refractive error according to the types but presented

the results for the total refractive error On this basis the prevalence of total refractive error

(23 for dyslexic and 225 for the control group) in the present study is similar to reports by

other studies Hoffman83 reported a 215 prevalence of refractive error in a study of the

incidence of vision difficulties in learning disabled children Although there was a marked

difference in the sample size in both studies the similarities in the findings may be because as

with the present study Hoffmans study was based on a population of learners in a special school

110

referred for optometric care by educators psychologists and reading specialists

Similar to the present study Evans et al 5 and Ygge et al 2 reported that the prevalence of

refractive errors in the dyslexic and control groups was similar Metsing and Ferreira103 found

no statistically significant difference between the learning disabled and the mainstream

groups in their study Contrasting findings were reported by Sucher and Stewart 98 who found

the prevalence of refractive errors to be 94 for the control group although the 208 found in

their dyslexic group is similar to the 23 found in the present study Wesson 146 reported the

prevalence of refractive error in children with reading dysfunction to be 38 while the

prevalence of refractive errors was 16 in the study by Sherman 84 In the study by Alvarez and

Puell 88 the prevalence of refractive error was 424 in the dyslexic group while 415 of the

control group had refractive error When compared to the present study the higher prevalence of

refractive error in the dyslexic group can be accounted for by the relatively high prevalence of

hyperopia (287) and in the control group the higher prevalence of refractive error can be

accounted for by the high prevalence of myopia (194) It is unclear why the prevalence of

hyperopia was as high as no information was given on how refractive error was assessed

Statistically for the present study the mean refractive errors for the dyslexic group were right

eye 086 098 left eye 057 1013 and right eye 070 103 left eye 049 109 for the

control group There was no statistically significant difference (RE p =066 LE p = 092)

Evans et al 91 reported a mean spherical refractive error of 077DS (right and left eye similar) In

the study by Alvarez and Puell 88 the refractive error results (descriptive statistics) reported

were as follows for the poor readers mean spherical equivalent refractive errors for the right and

left eye were 020 06 and 020 06 respectively In the control group mean spherical

equivalent refractive errors for the right and left eye were - 020 08 and - 014 08

respectively The descriptive statistics on refractive errors was not reported in the studies 3 93

cited earlier which made it impossible to compare the statistical findings with the present study

In the present study about 68 of participants from both groups were emmetropic (defined as lt -

050DS +100DS and 075D cyl) while 655 of the dyslexic children and 645 of children

from the control group were emmetropic in the study by Alvarez and Puell 88 Only the study by

111

Alvarez and Puell 88 reported on emmetropia

Refractive errors are among the leading causes of visual impairment worldwide and are

responsible for high rates of visual impairment and blindness in certain areas School children

are considered a high risk group because uncorrected refractive errors can affect their learning

abilities as well as their physical and mental development147

Several factors may lead to variations in results found in different studies This include the type

of population studied (clinical or non-clinical) sampling method (convenience or cluster)

classification criteria examiner bias and more specifically the use of cycloplegia in the

assessment of refractive errors

The following studies2 3 8895 categorized refractive errors as hyperopia myopia astigmatism

anisometropia or amblyopia

a Hyperopia In the present study hyperopia appeared slightly more prevalent (65) among

the dyslexic group than in the control group (3) This result agrees with the report from

other studies 93 95 and Alvarez and Puell88 who reported a much higher prevalence (287)

for the dyslexic group compared to 161 of the control group Helveston85 found no

difference in the prevalence of hyperopia between the population of normal and poor

readers

Hyperopia is the refractive error that is consistently reported 3939598 to be associated with

reading difficulties so it was expected that one will find the higher prevalence of hyperopia

in the present study

In the present study two children had latent hyperopia (based on the assumption that

increased plus did not blur the distance vision) The full magnitude of the hyperopic

findings could not be estimated because cycloplegia was not used Therefore it is possible

that the prevalence of hyperopia may have been under-estimated Williams et al 148 studied

the ldquoprevalence and risk factors for common vision problems in childrenrdquo and concluded

112

that the 48 prevalence of hyperopia in their study was ldquoan underestimate as data was

collected without cycloplegiardquo Borghi and Rouse (cited by Scharre and Cree)149

determined that on an average 063D more plus was identified by cycloplegic refraction

In relating hyperopia to reading difficulties simple to moderate hyperopia may not cause

constant blur at a distance or near point but the extra accommodative effort produces

asthenopic symptoms of intermittent blur headache fatigue and loss of concentration and

inattention in some patients which may be mistaken for short attention span Uncorrected

hyperopia is associated with esophoria at near point which can stress the fusional vergence

systems that hold the eyes in correct alignment If the hyperopia and esophoria is excessive

an accommodative esotropia can result 81150 This may result in difficulty in reading

b Myopia The 65 prevalence of myopia was the same in both groups This result is similar

to an equal prevalance of myopia reported by Eames 95 (4 in both groups) Alvarez and

Puell 88 reported that 57 of dyslexics were myopic while 194 of the control group was

myopic The prevalence of myopia was higher in studies by Rosner and Rosner 93 (54

non- learning disabled and 19 learning disabled) and lower than the prevalence reported

by Grisham and Simons150 In contrast to the present study subjects from the study by

Rosner and Rosner 93 were patients of an eye clinic where the two main reasons for referral

were reduced visual acuity and school learning problems while subjects for the present

study were referred to the special school mainly due to low academic performance which

may not necessarily be vision- related The subjects were chosen for the study irrespective

of whether they complained of a visual impairment or not whereas in the study by Rosner

and Rosner 93 study subjects had been identified as having a visual impairment Therefore

the higher prevalence may have been from the selection protocol used by Rosner and

Rosner 93 in their study

The prevalence of myopia was not reported in several studies 3568498146 referenced but was

presented as ldquorefractive errorsrdquo (was not classified as myopia hyperopia or astigmatism)

The onset and progression of myopia may be influenced by factors such as environment

nutrition genetic predisposition premature and low birth weight the effect of close work

113

racial and cultural factors differences in pupil size and illumination143151 Variations in the

prevalence of myopia reported by different authors may be related to the variation in the

above-mentioned factors

c Astigmatism

The prevalence (13) of astigmatism in the control group was higher than in the dyslexic

group (10) and there was no statistically significant difference between the two groups

Similarly Alvarez and Puell 88 found a higher prevalence of astigmatism in the control

group compared to the dyslexic group astigmatism was detected in 161 from the control

group compared to 8 and 92 right and left eyes respectively from the dyslexic group

In contrast Latvala et al 3 reported that 36 in the dyslexic group had astigmatism and

none from the control group had astigmatism Ygge et al 2 reported that the prevalence of

astigmatism was higher in the dyslexic group (28 left eye 25 ) compared to the control

groups (right eye 183 left eye 243) although there was no statistically significant

difference between the groups (p=025)

Rosner and Rosner93 reported that 30 of non-learning disabled and 27 of learning

disabled participants had astigmatism which differed markedly from the findings in the

present study despite the similarities in the study population Eames95 found an equal

prevalence of astigmatism (7) between the two groups studied

Astigmatism affects vision in different ways Clinically astigmatism over 150 diopters can

often cause severe eye strain and interfere with reading and even lesser degrees of

astigmatism can be symptomatic in some patients 81

Naidoo et al 157 studied refractive error and visual impairment in African school children in

South Africa Although the study was conducted specifically on mainstream school

children it relates to the present study as it provided useful information on the visual

characteristics of school children in South Africa and forms a basis for comparison since all

the studies reviewed in the present study were conducted on Caucasian population except

114

for the study by Metsing and Ferreira 103 which did not provide much information on

refractive errors in the their study of learning disabled children in Johannesburg South

Africa

Naidoo et al 157 reported a prevalence of hyperopia of 18 myopia of 29 astigmatism

of 67 and 68 right eye and left eye respectively In comparison the prevalence of

refractive error for the control group in the present study was hyperopia 3 myopia 65

and astigmatism 13 The refractive error trend (astigmatism being higher in prevalence

than myopia and hyperopia) was similar between the present study and the study by Naidoo

et al 157 despite the difference in prevalence reported Given that ethnic origins culture

socio-economic class are comparable between both studies the difference in prevalence

may (partly) have been due to different sampling methods (cluster versus convenience) and

the use of cycloplegia in their study The use of cycloplegia has been reported to yield

spherical aberrations and unpredictable errors due to associated mydriasis 110

It has also been documented 158 that the use of cycloplegia yields more positive retinoscopic

findings although this did not seem to apply in this comparison as the prevalence of

hyperopia in the present study was higher than that reported by Naidoo et al 157 A possible

explanation for the difference in prevalence may be related to the different criteria used to

define hyperopia Bennet et al 159 stated that the degree of ametropia ldquohellip considered to be

within normal limits in a particular study will drastically affect the prevalencerdquo Similar

opinion was noted by Gronlund et al 140 who stated that ldquothe choice of definition will of

course have a great impact on prevalence In their study on Swedish children aged 4-15

years Gronlund et al 140 defined astigmatism as ge 075 and stated that the prevalence of

astigmatism would have been 21 (instead of 32) had they defined astigmatism as ge

100 Furthermore Grisham et al 86 in their ldquoStudy of poor readers in high schoolrdquo noted

that helliprdquoif one selected less stringent criteria then the proportion of students with any given

dysfunction would decreaserdquo In the present study the criteria used to classify hyperopia

were ge +100D while Naidoo et al 157 used ge +200DS cut- off

115

d Anisometropia Anisometropia may be defined as a difference of 075 (sphere or cyl) or

more between the two eyes and this definition was the criteria used in this study 129

Anisometropia is of great clinical interest because of its intimate association with

strabismus and amblyopia141 152

The 65 prevalence of anisometropia was similar in both groups This is comparable to

results reported by Latvala et al 3 where the prevalence of anisometropia was 36 (2

subjects) for the dyslexic group and 6 (3 subjects) for the control group but contrasts with

findings by Eames 95 who reported the prevalence of anisometropia to be 13 for poor

readers and 6 for the control group while Ygge et al 2 found a higher prevalence of

anisometropia in the control group (158) than in the dyslexic group (94)

In anisometropia the difference in refraction as well as the refractive error causes the image

to be out of focus on one retina blunting the development of the visual pathway in the

affected eye 141 The fovea of an anisometropic eye receives images from the same visual

object however the images from the more myopic or hyperopic eye are out of focus 153 It

appears that anisometropia is a major cause of amblyopia for at least one third of all

amblyopias 131 and anisometropia is considered to be a causal factor in the pathogenesis of

amblyopia and strabismus It is also estimated that 6 to 38 of all cases of amblyopia are

caused by anisometropia without strabismus whereas in about 12 to 18 of the children

with strabismus this is accompanied by anisometropia 154

According to Grosvenor 114 uncorrected anisometropia even of a moderate amount may

induce eyestrain because it is impossible for the accommodation mechanism to maintain

clear images on the retina at the same time On the other hand large amounts of

anisometropia (2DS or more) may not cause symptoms as no effort is made to maintain

single binocular vision 114

Rutstein and Corliss155 investigated the relationship between anisometropia amblyopia and

binocularity and concluded that higher degrees of anisometropia generally cause deeper

116

amblyopia and poorer levels of binocularity for hyperopes but not for myopes and stated

that ldquoAlthough uncommon small amounts of anisometropia can cause moderate

amblyopiardquo However the results of the present study failed to support this claims as none

of the subjects had amblyopia resulting from anisometropia

In anisometropia the displacement or distortion of the image prevents the development of

fine visual perception in the occipital cortex and puts the child at risk for developing

amblyopia 141 Anisometropia causes poor reading skills probably through the mechanism

of poor sensory and motor fusion rather than reduced visual acuity It degrades binocular

coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 150

e Amblyopia Two subjects (65) in the dyslexic group had ambyopia due to cataracts and

none from the control group had amblyopia This result may be comparable to studies by

Latvala et al 3 who reported 36 (2 subjects) for the dyslexic group and that of Rosner and

Rosner 93 with a prevalence of 4 for the non-learning disabled group and 3 for the

learning disabled group Cataract is a major cause of amblyopia (deprivation ambyopia) by

causing an impediment to the visual axis 141 Other conditions that may lead to amblyopia

include strabismus (which causes the images on the retinas to be dissimilar) and uncorrected

refractive errors (especially as in anisometropia) Anisometropia is related to amblyopia

Amblyopia is a major public health problem It is the most common cause of monocular

vision loss in children and young adults Early recognition and prompt referral are crucial

especially during infancy and childhood to prevent permanent loss of

vision 141

523 Heterophoria

Heterophoria is the relative deviations of the visual axes when the eyes are dissociated 160 Some

degrees of heterophoria are considered normal for persons with normal binocular vision

Approximately 1-2 prism dioptres of esophoria or 1- 4 diopters of exophoria at distance should

117

be considered to be within normal limits161 At near 3-6 prism dioptres of exophoria is

considered normal (physiological exophoria) 161 A normal healthy eye is usually able to

overcome these small deviations and so it is described as being compensated161 However

esophoria is much less compatible with comfortable vision than is exophoria Any amount of

esophoria in a patient with visually related symptoms may be problematic 162

In the present study all subjects were orthophoric at distance This report corroborates reports by

other authors Mathebula et al 163 studied heterophoria in a South African population of school

children (mean age 10 16 years) and reported that the mean distance horizontal heterophoria

showed orthophoria Similarly Evans et al 100 reported that all subjects examined ldquowere

orthophoric at distance with cover testrdquo According to Dowley160 ldquothe high prevalence of

orthophoria is realrdquo and that ldquothe high prevalence of distance orthophoriahellip may be due to the

coordinating influences of the same mechanisms that subserves prism adaptationrdquo A similar

view was expressed by Walline et al 164 who studied the ldquodevelopment of phoria in childrenrdquo and

stated that ldquonearly all of our subjects were orthophoric at distancerdquo

The major findings in heterophoria was a 95 prevalence of exophoria at near which was more

in the dyslexic subjects than in the control group with no exophoria There was no statistically

significant difference between the two groups (p= 059) At near 3 (1 subject) of the dyslexic

group had an esophoria greater than or equal to four prism diopters An interpretation of the

above finding is that the higher prevalance of heterophoria may mean that the dyslexic subjects

may be more uncomfortable when doing near work than the nroaml readers A possible

explanation may be that according to von Noorden (cited by Kommerell et al)165 heterophoria

typically causes asthenopia Patients with asthenopic symptoms (feeling of heaviness dryness

and soreness of the eyes pain in and around the eye frontal and occipital headache and the eyes

are easily fatigued) often have an aversion to reading and studying Typically such complaints

tend to be less severe or to disappear when patients do not use their eyes in close work165 It has

been documented that decompensated heterphoria (heterophoria accompanied by symptoms) can

be due to poor fusional reserves problems with sensory fusion (such as in anisometropia) or may

be due to an unusually large phoria which may be due to anatomical reasons or uncorrected

hyperopia 161166 The presence of phoria could contribute to visual and attention abnormalities

118

noted among children with reading difficulties making near point visual activities more

strenuous

Nearpoint esophoria has been reported to be associated with childhood myopia progression 164167

In the present study the only subject that had four prism diopters esophoria at near in the

dyslexic group had +125 hyperopia while the subject that had 3 prism diopters esophoria had -

150 DS myopia However due to the small sample size in the present study it is difficult to

draw any conclusion on the association between myopia and esophoria

The findings on heterophoria as in other aspects of vision functions in the dyslexic populations

reviewed are mixed However similar to the present study Latvala et al 3 assessed near

heterophoria using the Maddox Wing and reported a higher prevalence of near exophoria in the

dyslexics than in the control group Similarly Kiely et al 12 documented that dyslexics tend to

be more exophoric than normal readers Evans et al 101 found a higher prevalence of lateral

phoria in learning disabled subjects compared to normal readers In contrast to the above

findings Sucher and Stewart 98 assessed phoria using the cover test (testing distance was not

indicated in the results) but found a slightly higher prevalence of phoria in the control group than

in the dyslexics Evans et al 100 assessed near horizontal phoria using the Maddox Wing as in

present study but found no difference in near horizontal phoria between the dyslexic and control

groups Bucci et al 89 reported that there was no difference in phoria results at both distance and

near between the dyslexic and control groups Metsing and Ferreira 103 found no statistically

significant difference between the learning disabled and the mainstream groups on

heterophoriaThe use of the cover tests for both distance and near to assess heterophoria may have allowed for

standardization in technique but may not have implied that more valid results will be obtained

The cover test and the Maddox Wing have been used simultaneously to assess heterophoria in

other studies3 100 The possible sources of variations in results reported by different investigators

include classification criteria and poor technique 168

524 Near Point of Convergence

119

In the present study the prevalence of a remote (greater than 10cm) near point of convergence

was higher in the control group with 48 than in the dyslexic group with a prevalence of 33

There was a statistically significant difference in the near point of convergence break (p=0049)

and recovery (p = 0006) between the two groups

It may be that children who do not have reading difficulties tend to read more often than children

who experience difficulty to read This is because with increasing ability to read there is likely

to be more demand on accommodation and convergence resulting in near point stress as well as

esophoria The normal readers use and exercise the accommodation and convergence system

more frequently According to Owens and Wolf-Kelly 169 near work has long been considered a

potential source of visual problems stating that in their study reading ordinary text at a near

distance for about one hour induced significant changes in the resting postures of both

accommodation and vergences The authors indicated that ldquonear work induces a recession of the

near point of accommodation or vergencesrdquo

The near point of convergence break is associated with changes in inter-pupillary distance with

age As inter-pupillary distance widens with physical growth the amount of convergence in

prism diopters increases for a given distance from the subject 132 Chen et al 170 studied near point

of convergence in children aged 1-17 years and reported that an increasing incidence of remote

near point of convergence with increasing age in their study might be due to the near work

demands of primary school which might create a different level of near point stress than the near

work conditions in pre- primary school years

Similar to the findings in the present study Evans et al 91 reported a statistically significant

relationship between NPC and reading retardation (p=0019) Contrasting findings were reported

by other authors Bedwell et al 171 studied visual and ocular anomalies in relation to reading

difficulties and reported that 466 of the group of ldquogood readersrdquo had abnormal near point of

convergence compared to 58 of ldquopoor readersrdquo Similarly Bucci et al 89 and Kapoula et al 87

reported that the near point of convergence was significantly more remote in dyslexics than the

control In the study by Latvala et al 3 1275 of the dyslexic group and only 2 from the

120

control group had a near point of convergence of worse than 8cm A statistically significant

difference between the two groups was reported (p=00385) Metsing and Ferreira 103 found

no statistically significant difference between the learning disabled and the mainstream

groups on the near point of convergence function In a study that lacked control subjects

Grisham et al 86 reported a lower prevalence (154) of NPC (9cm or further) in a group of high

school poor readers The authors measured NPC three times on each subject in other to detect

fatigue but stated that it should therefore be noted that fatigue may be a problem for poor

readers even if they are able to converge at 8 cm if they tire easily then the ability to sustain

convergence during longer periods of reading may suffer

In the present study the measurement of the near point of convergence was repeated three times

on each subject in order to detect visual fatigue that the children experience everyday This

approach was documented by other authors 868789 111-113 Some authors111-113 have recommended

the measurement of the near point of convergence several times in order to detect fatigue which

is often indicative of poor convergence fusion system which may affect distract a child from

reading 172

525 Accommodation Functions

a Accommodation Facility Accommodative facility assesses the rate at which

accommodation can be stimulated and inhibited repeatedly during a specific time period It

relates to the individualrsquos ability to shift focus quickly and efficiently for varying distances

and is extensively used in the reading process 6

Only the result for the binocular accommodative facility was recorded and was used for analysis

in the present study Siderov and Johnston 174 noted that monocular accommodation

measurements provides a direct evaluation of the dynamics of accommodative response while

binocular testing of accommodative facility provides similar information but also reflects the

interactive nature of the relationship between accommodation and vergences

For the dyslexic group 54 (15 subjects) had inefficient accommodation facility while only 33

121

(9 subjects) of the control group had inefficient accommodative facility Statistically the control

group had a better accommodative facility (885cpm 369) than the dyslexics (686cpm 274)

and there was a statistically significant difference between the two groups (p =0027) It has

been documented 100 that an abnormal accommodative facility could imply difficulty in changing

focus from far to near and subsequently lead to a lack of interest in learning An efficient facility

of accommodation is particularly important because a greater period in school involves changing

focus between the chalkboard to near task such as writing at a desk Low values of

accommodative facility have been associated with symptoms related to near point asthenopia134

The result of the present study corresponds with findings by Evans et al 100 who reported that

dyslexics appeared to be slower at a test of accommodative facility Similar results were reported

in other studies 128898 but contrasts with results reported by Buzzelli 6 Buzzelli 6 assessed

accommodation facility using a Bernell Vectogram that utilized a polarised target to control for

suppression Buzzelli6 assessed accommodation facility as follows patients were asked to clear

the number 9 on the Bernell Vectogram target at 40cm They alternately viewed the target

through a 2D lens The test was continued for 20 cycles (a cycle being clearing the plus and

minus lens each presented once) The total time in seconds was recorded for the right eye left

eye and both eyes The polarized target controlled for suppression The use of the vectogram

to assess accommodation facility by Buzzelli6 differs from the flipper technique that was used in

the present study Secondly there was no control for suppression in the present study The

difference in results found between the two studies may be due to the lack of control for

suppression in the pre sent study Siderov and Johnston174 stated that ldquoclinical measurements of

binocular accommodation facility could vary depending on whether or not suppression has been

monitoredrdquo

A total of 317 of the poor readers in the study by Grisham et al 86 had inadequate (9cm or less)

accommodative facility

Allison173 suggested that there are different norms submitted by different authors for

accommodative facility measurements and that apart from adhering strictly to recommended

norms for accommodative facility testing other factors to be considered when evaluating

accommodative facility include the difficulty and speed of responses to the plus and minus lenses

122

and whether the subjects fatigue easily 173 Kedzia et al (cited in Zadnik)175 documented that

the factors that affect the testing of accommodative facility include the size and position of the

text being read the complexity of the target reaction time of the child to call out the symbol and

the magnification or minification factors introduced by the lenses themselves The above

variables are factors to be considered in assessing the variations in the findings for

accommodative facility reported in this study as compared to other studies

b Amplitude of Accommodation Accommodation is the ability to adjust the focus of the

eye by changing the shape of the crystalline lens to attain maximum sharpness of the retinal

image of an object of regard The absolute magnitude of the accommodative response is

termed the accommodative amplitude 120

Statistically the dyslexic group had a slightly reduced monocular amplitude of

accommodation (mean 12D) compared to the control group (1287D) but there was no

statistically significant difference between the two groups (p=070) The slight difference

in the mean amplitude of accommodation between the two groups may have been due to the

two subjects from the dyslexic group who had latent hyperopia

Similar to the present study lvarez and Puell88 reported that monocular accommodative

amplitude was significantly lower in the group of poor readers Evans et a l91100 in two

seperate studies 91100 reported that amplitude of accommodation was reduced in the

populations studied Ygge et al 102 and Goulandris et al 90 found no statistically significant

difference between the dyslexic and control groups Grisham et al 86 found that 247 of

the children had amplitude of accommodation which the authors classified as ldquoweakrdquo or

ldquovery weakrdquo while 115 had accommodative amplitudes classified as ldquoborderlinerdquo In the

present study the mean amplitude of accommodation for each group was within normal age

norms according to the amplitude norms and Hofsteterrsquos formula 106161

In the study by Metsing and Ferreira 103 a higher percentage of low amplitude of

accommodation for the right and left eyes respectively (516 and 533) was found in the

mainstream group compared to 291 and 288 in the learning disabled group The

123

relationships between the mainstream group and reduced amplitudes of accommodation of

the right (p=004) and left eyes (p = 0001) were found to be statistically significant

(plt005) Furthermore the relationship between the mainstream group and reduced ampli-

tude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to be moderate and

that between the mainstream group and the right eyes (Cramerrsquos = 0286) to be low

When accommodation amplitude is assessed monocularly it measures the response for each

eye individually and the limiting factor is the magnitude of blur-driven accommodation

Monocular amplitude measures are particularly important to determine whether a patient

has accommodative insufficiency 107 The amplitude measured binocularly is usually

greater than the monocularly measured amplitude due to the influence of both the blur-

driven accommodative response and accommodative and vergence responses 120 175

Clinically the amplitude of accommodation is considered important mainly when it falls

below the expected age norm in which case the child may experience blur vision at near

Secondly a difference of up to 2D between the two eyes may also be considered clinically

significant 120

An inefficient accommodation function may lead to difficulties in learning as the focusing

system of the eyes play a major role in the learning process Children who suffer some

anomalies of accommodation are more prone to fatigue quickly and become inattentive than

those who have norrmal accommodation function 87 Symptoms of accommodation

insufficiency are specifically related to near vision work 176

Factors affecting the measurement of amplitude include differences when measurements

are taken monocularly as compared to when assessed binocularly the angle of gaze target

size age refractive error race and climate 175

c Accommodation Posture During near vision the eyes are not usually precisely focussed

on the object of regard but the accommodation lags a small amount behind the target If

the accommodation lag is small then the blur it causes is insignificant if high then it can

result in blurred print during reading 142

124

About 3928 (11 subjects) of dyslexic subjects had lag of accommodation compared to

4193 (13 subjects) of the control group The mean accommodation lag was similar in

both groups 092 for control and 088 for the dyslexic group There were no statistically

significant differences between the two groups right eye (p = 083) and left eye (p = 060)

Similarly Evans at al 100 reported that the mean accommodation lag did not show any

statistically significant difference between the dyslexic and control groups On the

contrary Metsing and Ferreira 103 found a high prevalence of lead of accommodation for

the mainstream group but a high prevalence of lag of accommodation in the learning

disabled group The relationships were found to be statistically significant (plt005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000) In another

study Evans et al 91 reported a mean accommodation lag of + 112DS

An individual with a lag of accommodation habitually under accommodates and may lead

to difficulty with reading An accommodative response that manifests as an excessive lag

of accommodation may indicate latent hyperopia esophoria or may be associated with

accommodative insufficiency or accommodative spasm poor negative vergences or when

patient is overminused 177 The prevalence of high lag may be related to the prevalence of

latent hyperopia and esophoria in the present study

C Relative Accommodation (PRA NRA) The relative accommodation tests assess the

patientsrsquo ability to increase and decrease accommodation under binocular conditions when

the total convergence demand is constant67 It is also an indirect assessment of the

vergence system since the vergence demand remains constant while accommodative

demand varies 121 The results for the relative accommodation for all subjects from both

dyslexic and control groups were unexpectedly high The norm for the mean NRA and PRA

were given as approximately +2 or - 2 diopters from two reports consulted 119 121 while the

relative accommodation results in this study ranged between 3 and - 6 diopters Similarly a

higher relative accommodative value was reported by Chen et aland Abidin139 in a study of

vergence problems in Malay school children The only available study accessed that

125

reported on relative accommodation was conducted by lvarezand Puell 88 who reported

that the negative and positive relative accommodation values were similar in both groups of

children Statistically the results were (NRA Control group 19 06 poor readers 19

06 PRA Control group 23 09 Poor readers 20 13) compared to the findings from

the present study (NRA Control group was 311 047 NRA dyslexic group 322 079)

(PRA Control ndash 606 063 PRA dyslexic was -623 117) The p values were not

indicated

The higher values found in the present study may be due to the process of addition of trial

lenses specifically The phoropter has been reported to be a better technique to assess

relative accommodation 119

Although the relative accommodation was not assessed by many authors different opinions

were expressed regarding the assessment of relative accommodation Latvala et al 3 noted

that positive and negative relative accommodation ranges are ldquodifficult to take and

unreliablerdquo According to Morgan 178 the PRA and NRA results are often unusually high

but recommended that such tests results can be used as a sort of indicative way and can only

be interpreted loosely and suggested that the use should be restricted to special

circumstances Garcia and Fransisco 122 reported that high NRA can be associated with

disorders such as accommodative insufficiency and convergence excess while high values

of PRA are related to anomalies in which accommodative excess appears but concluded that

PRA and NRA findings were used mainly as complementary diagnostics tests of some

disorders in literatures consulted

A high value of the negative relative accommodation could indicate that the children were

exerting maximum accommodation effort Because 250D of accommodation is exerted at

the 40cm working distance the maximum amount of accommodation that would be

expected to relax accommodation at 40cm would be 250D A value greater than 250D

would mean that accommodation may not have been fully relaxed during the subjective

refraction 114 Generally a high value of the NRA may also mean that the refraction may

126

have been under corrected for hyperopia or over corrected for myopia 122 Garciaand

Francisco122 reported the relationship between high NRA values and under corrected

hyperopia to be due to the etiology of hyperopia

According to Grosvenor 114 the amplitude of accommodation is a limiting factor for the

expected value of the PRA For example ldquoIf a patient has an amplitude of accommodation

of only 150D we would expect the minus-lens-to-blur test to be no greater than -150Dhelliprdquo

A high value of the positive relative accommodation is related to convergence insufficiency 122 Illumination and the range of fusional vergence also affect the relative accommodation

An excessive illumination will give an erroneously high finding due to the increased depth

of focus 100

Dysfunctions of accommodation can significantly interfere with the comfort clarity speed

and accuracy of reading as the child develops reading skills 51 Wick and Hall 120 suggested

that the results for the tests of accommodation are more meaningful when analyzed

together as the results of individual accommodation function may not give a true reflection

of the childrsquos accommodation dysfunction

526 Fusional Reserves

Fusional vergence amplitudes is a clinical measurement of a person`s fusional vergence ability It

reflects the ability of the oculomotor system to maintain sensory fusion in spite of varying

vergence requirements 179 The horizontal vergence reserves describe the amount by which the

eyes can be converged or diverged whilst the subject maintains fusion 100161 It is a measure of

how much fusional vergence is available in reserve that can be used to overcome a phoria The

amount of base out prism required to produce diplopia is called positive fusional reserves

(measures convergence) 100 The measurements of convergence fusion are also referred to as the

lsquoamplitude of fusional reserves 111 Clinically the fusional amplitude provides information about

a patientrsquos ability to maintain comfortable binocular vision 100112

The participants from both the dyslexic and control groups either could not report or understand

127

blur so the result for break and recovery was used in all analysis of vergence function Evans et

al 100 reported similar observations For the fusional reserves only the findings for the near

positive fusional vergence (convergence) are emphasized The positive fusional

reservesvergence (measures convergence ability) may be more important in assessing reading

dysfunction More so it has been suggested by other authors 112121180 that near measurements

are more relevant in assessing vision functions in dyslexic children

Based on Bishops 111 recommended norm of between 30 and 40 prism diopters for positive

fusional amplitude at near the prevalence of abnormal positive fusional vergence at near was

83 for the control and 74 for the dyslexics This appears unusually high Walters 168

explained that a possible reason for an unexpected high base out finding is that exophoric

conditions makes compensating for base out more difficult than base in testing It is unclear if

this is the case in the present study although the prevalence of exophoria at near in the present

study was 95 The compensation for a phoria is maintained by fusion reflex so knowing the

magnitude of the vergence amplitude that is needed to compensate a given phoria is very useful

clinically181

For the present study at near the negative fusional vergence (base in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base out

vergences) was similar for both groups Overall there was no statistically significant difference

between the groups (p = 021 for BI break at near) and (p = 084 for BO break at near) Evans et

al 100 reported that the dyslexics have reduced fusional reserves compared to the control group

Bucci et al 89 reported reduced divergence capabilities in dyslexics at near

When assessing fusional reserves the blur point is a function of the flexibility between vergence

and accommodation When little flexibility exists the addition of even low prism causes a

simultaneous accommodation shift with resultant blur Lack of low blur findings may indicate an

inadequate flexibility bond between accommodation and convergence The break point is an

indication of the quality of binocular function Break values will be adequate when fusion ability

is good while the recovery measure is a more subtle indicator of the quality of binocular function

Consequently reduced blur break or recovery findings indicate the presence of near point stress

128

182 Some subjects from both groups had break values higher than 40 prism diopters for base in

and base out reserves Wesson et al 125 in their study reported a similar observation on objective

testing of vergence ranges Such high break findings may be because suppression was not

controlled while assessing the fusional reserves in the present study When suppression is

controlled the average vergence values will be lower because the test is stopped when the

suppression is detected If suppression is not monitored the break is not detected until the

stimulus is outside the suppression zone and a higher vergence value is obtained 125 However it

is important to note that the significance of such suppression in binocular individuals is unknown 126 Again in special cases such as suppression large abnormal findings (compared to

normative ranges) may permit better functions in reading than a lower abnormal finding 183 For

example an individual who totally suppresses the vision of one eye is less apt to have difficulty

at reading than the individual who only partially suppresses the vision of one eye 183

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups The

mean fusional convergence capacities at distance were 1680 prism dopters for both the control

and the dyslexic group At near the corresponding figures were 2640 prism diopters and 2670

prism diopters respectively The mean fusional divergence capacity at distance was 650 prism

diopeters and 620 prism diopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 1050 and 1020 prism diopters respectively There was no

statistically significant difference between the groups

Latvala et al 3 reported a prevalence of fusional amplitude greater than or equal to 32 prism

diopters to be 61 for the control group and 75 for the dyslexic group using referral criteria

of 32 prism diopters at a distance of 33cm At near a testing distance of 40cm was used in this

study The difference in prevalence between the two studies might be from the different test

distances used Furthermore as detailed by Rouse et al 158 large intra subject intra-examiner

and inter-examiner variations in the assessment of fusional reserves have been reported

Vergence amplitudes have been reported to vary with alertness that is whether the subject is

tired or rested or under the influence of a toxic agent 184

129

Bedwell et al 171 reported that neither convergence nor divergence were significantly related to

reading difficultyrdquo Similarly Metsing and Ferreira103 found no statistically significant

difference between fusional vergences amplitude (Base-out base-in) at near No details were

provided

There are several limitations with the assessment of fusional reserves Sheedy (cited by Rouse et

al) 158 remarked that ldquo the general opinion is that when fusional vergence tests are repeated on

the same patient the second value found may be quite different from the first a difference of 10

PD from one fusional vergence amplitude measurement to another is not unusual unless rigorous

controls ar a typical difference of 3- 4 prism diopters but can measure differences as large as 12

prism diopter on follow up visit and that inter-examiner variation can be as large as 10 to 16

prism diopters The authors concluded that the positive fusional vergences difference could be

due to children having difficulty understanding the instructions or expected endpoints children

being slower responders or may be poorer observers The blur readings on the vergence reserves

for both distance and near were excluded in the analysis as it was difficult to get the children to

elicit a proper response It is possible that the children did not understand the instruction or the

concept of blur despite repeated explanations and trial readings This variable is often difficult

for young subjects to understand Similar problems were encountered by Evans et al 100 who

commented that several subjects were unable to appreciate a blurrdquo Scheiman et al 126 stated

that ldquowe were unable to consistently obtain a blur finding from our samplerdquo and that rdquo less

than13 of the subjects were able to report a blur we therefore only recorded the break and

recovery findingsrdquo

Wesson et al125 emphasized that the ldquomajor limitations of any vergence measurement

isrdquohellipnecessity to obtain a subjective response to blur break and recovery and that working with

children whose responses are not reliable is difficultrdquo It was suggested that a break and recovery

point could be determined by observing the subjects eye movements as prism power is increased

This technique was applied in this study However most children have difficulty maintaining

fixation long enough to measure vergence ranges

Vergence is influenced by several factors including awareness of the distance of the object

130

(proximal vergence)cross-linking with the accommodative system (acommodative vergence) and

the fine tuning of ocular alignment during the fusion of each monocular image into a single

percept (fusional vergence) 161 and other factors such as test target and lighting conditions 125

Generally in the assessment of a patientrsquos binocular status the fusional reserves may be more

useful when analyzed with the phoria measurements 115

The vergence system is closely related to the accommodative system and symptoms may at times

appear similar The symptoms associated with deficiencies with the vergence system include

letters or words appear to float or move around postural changes noted when working at a desk

difficulty aligning columns of numbers intermittent diplopia at either distance or near 185

Furthermore ldquodysfunctional vergence could cause an excess of eye movements especially of

small saccades The ability to continue the rapid decoding of the visual characters decreases as

more stress is placed on the vergence mechanism of the dyslexic during reading which makes

them tire more quickly than normal readersrdquo 6

A possible explanation for the relationship between vergence control and reading proposed by

Riddel et al (cited by Morad et al) 186 is that children with poor vergence control had impaired

accuracy of spatial localization that may impend their ability to accurately determine the position

of letters within words

527 Ocular Pathology

Two participants from the dyslexic group had cataracts Only the study by Hoffamn 83 reported

that one child had conjunctivitis

54 Summary

The results of this study do not vary significantly from the international studies in spite of the

differences in study designs tools and sample sizes Although studies have indicated differences

131

in the prevalance of open angle glaucoma and myopia between Caucasian and African

populations this study does not indicate a specific difference in the prevalance of vision variables

between Caucasian and African dyslexic school children However the lack of studies on

African children makes it difficult to reach any conclusions regarding structural differences that

may affect the presence of dyslexia Comparing the findings of this study with those done

internationally highlights the complexity of investigating the visual conditions that may be

associated with dyslexia and the need for rigorous and consistent testing methods in order to be

able to compare results

Having completed a discussion of the study findings in Chapter Five Chapter Six will present the

study conclusion and indicate its limitation as well as recommendations for future studies

132

CHAPTER SIX CONCLUSION

61 Introduction

Being able to read and write is an essential part of being able to interact with the world As

vision plays a major role in reading and the learning process a normally working visual system is

essential for efficient reading

With as many as 20 of Caucasian children being affected to a greater or lesser degree by

dyslexia 1631 there is a need to identify its cause and develop remedial actions to reduce its

effects As studies 4 59 have shown that intellectual ability is not the cause of dyslexia research

to enable people to reach their full potential in spite of their impaired reading ability requires a

multidisciplinary approach of which optometrists are part

This study aimed to determine the prevalence of vision conditions in a South African population

of African dyslexic children and to study the relationship between vision and dyslexia in an

African setting This was done by investigating visual acuity refraction and binocular functions

between two groups of 31 African school children one group attending a school for children with

learning difficulties and the other a mainstream school This enabled a study of possible vision

defects in African dyslexic children as well as a comparison between the two groups

The study targeted African school children its objectives being to

1 determine the distribution of visual acuity disorders among dyslexic children

2 determine the distribution of refractive errors among dyslexic children

3 determine the distribution of heterophorias among dyslexic children

4 determine the distribution of strabismus among dyslexic children

5 determine the distribution of accommodation disorders among dyslexic children

6 determine the distribution of vergence disorders among dyslexic children

7 determine the distribution of ocular pathology among dyslexic population

133

8 compare these findings to a similar group of non-dyslexic children

A review of the literature showed that most studies that investigated visual functions in dyslexic

school children were conducted on Caucasian population and that the findings of these studies

were inconclusive The broad areas of vision investigated in the previous studies 2-6 87-89 were

visual acuity and refraction binocular vision and ocular pathology

As studies conducted on the African population was lacking this study examined visual acuity

(using LogMar charts) refraction (static retinoscopy without cycloplegia) near point of

convergence (RAF rule) [accommodation functions 2 flipper lenses to assess accommodation

facility MEM retinoscopy for accommodation posture and pushndashup method to evaluate

amplitude of accommodation] ocular alignment (cover test and Maddox Wing) and strabismus

(Hirschberg test) and fusional reserves (using prism bars)

The study was conducted on a African population of 31 dyslexic school children selected from a

school for children with learning difficulties and 31 control participants from a mainstream

school in Durban Their ages ranged between 10-15 years The participants were selected using

the convenient sampling method as there were only a few participants classified as being

dyslexics All data collection procedures were conducted at the respective schools

The prevalence of vision conditions were presented in percentages () Data was analyzed using

the Statistical Package for Social Sciences (SPSS) and was expressed as mean (M) and standard

deviation (SD) and calculated for descriptive purposes at 95 confidence interval (CI) The level

of significance considered to support a hypothesis was taken as P lt 005 For comparison all

data from both groups was subjected to a two sample t test (2-tailed)

134

62 Summary of findingsIf an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants (control group) would be

expected

621 Visual Acuity

The prevalence of visual acuity worse than 69 (23 dyslexic and 225 control) is the same in

both the dyslexic and the control group and there was no statistically significant difference (p =029 023 right and left eye respectively This is similar to the findings by Buzzelli 6 and

Goulandris et al 90 which found no statistically significant difference between the two groups

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexia and the control group (p=029 for the right eye and 023 for the left eye)

622 Refractive Errors

The prevalence of refractive errors was similar in both groups There was no statistically

significant difference between the two groups (p=066 for the right eye and 092 for the left eye)

This is similar to the findings by Evans et al 2 which found no statistically significant difference

between the two groups (p=058)

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 066 for the right eye and 092 for the left eye)

a Hyperopia Hyperopia was more prevalent in the dyslexic group than in the control group

This is consistent with findings in other studies 88 93 95

b Myopia The prevalence of myopia was the same in both groups This is similar to the

findings by Eames 95 which found no difference in the prevalence of myopia between the

two groups

135

c Astigmatism Astigmatism was more prevalent in the control group than in the dyslexic

group This is similar to the findings by Alvarez et al 88 which found a higher prevalence of

astigmatism in the control group than in the dyslexic group

d Anisometropia The prevalence of anisometropia was the same in both groups (2 subjects

each) This is comparable to results reported by Latvala et al 3 where the prevalence of

anisometropia was 36 (2 subjects) for the dyslexic group and 6 (3 subjects) for the

control group

e Amblyopia Amblyopia was more prevalent in the dyslexic group compared to the control

group This is similar to the findings by Latvala et al 3 and Rosner and Rosner 93 which

found a higher prevalence of astigmatism in the dyslexic than in the control group

623 Near Point of Convergence

The prevalence of remote NPC (ge10cm) was higher in the control group than in the dyslexic

group and there was a statistically significant difference between the two groups (p= 0049) This

is similar to the findings by Kapoula et al 87 Latvala et al 3 and Evans et al 91 who found a

statistically significant difference between the dyslexic and the control groups on the NPC

functions

The null hypothesis was rejected as there was a statistically significant difference between the

dyslexic and control groups (p = 0049)

624 Heterophoria

The prevalence of exophoria at near was higher in the dyslexic group (95) than in the control

group with no exophoria There was no statistically significant difference between the two

groups (p= 059) The prevalence of esophoria in the dyslexic group was higher than that found

136

in the control group and there was no statistically significant difference (p =046) This is similar

to findings in other studies 3 12101163 which found no statistically significant difference between

the two groups

The null hypothesis was accepted as there was no statistically significant difference between the

dyslexic and control groups (p=059 for exophoria) and (p=046 for esophoria)

625 Accommodation Functions

a Amplitude of Accommodation The dyslexic group had a reduced monocular amplitude

of accommodation compared to the control group but there was no statistically significant

difference between the two groups (right eye p = 007 left eye p=022) This is similar

to the findings reported in other studies 3 87 91102 which found no statistically significant

difference between the two groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation amplitude (right eye

p=007 left eye p=022)

b Accommodation Facility The participants from the dyslexic group performed

significantly worse than the control group in accommodative facility function and there

was a significant difference between the two groups (p=0027) This result corroborates

findings reported in other studies 12 88 98100 which found a statistically significant

difference between the two groups

The null hypothesis was rejected as there was a statistically significant difference between

the dyslexic and control group in accommodation facility (p=0027)

c Accommodation Lag The prevalence of lag of accommodation was higher in the control

group compared to the dyslexic group and there was no statistically significant difference

There were no statistically significant difference between the two groups (right eye p=083

137

and left eye p= 060) This is similar to the findings by Evans et al 100 which found no

statistically significant difference between the two groups The mean accommodation lag in

the present study were 092D for control and 088D for the dyslexic which is comparable to

reports by Evans et al 91 who reported a mean accommodation lag of + 112D

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation posture

d Relative Accommodation (PRA NRA) The mean values for the relative

accommodation are similar in both groups and there was no statistically significant

difference between the groups (p=068 for NRA and p =051 for PRA) This is similar

to the findings reported Alvarez and Puell 88 which found no statistically significant

difference between the two groups in relative accommodation

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups in relative accommodation

626 Fusional Reserves

For the present study at near the negative fusional vergence (base-in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base-out

vergences) was similar for both groups There was no statistically significant difference between

the groups (p= 021 for BI break at near) and (p = 084 for BO break at near) This is similar to

the findings by Evans et al100 which found no statistically significant difference between the two

groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 021 for BI break at near) and (p = 084 for BO break at near)

627 Ocular Pathology

138

Two participants from the dyslexic group had cataracts

63 Implications of the Study Findings

a Causal Relationship This study presented the prevalence of some vision defects in a

South African African population of dyslexic school children as compared to a population

of non-dyslexic children and could not establish any causal relationship between vision and

dyslexia The vision parameters that showed that statistical significance may only be

reflective of factors associated with dyslexia but in a non-causal way This may mean that

the prevalence of the particular visual variable is much higher in the dyslexic population

than in the control group Vision is essential for reading but the vision anomalies that are

found to be associated with dyslexia also occur in children who do not have reading

difficulties It is therefore possible that other complex vision functions may be more related

to the etiology of dyslexia than peripheral visual factors such as NPC and accommodative

facility which showed statistically significant differences in the present study

b Risk Identification The findings of this study also suggest that dyslexic children are not

at more risk for the identified vision condition compared to non-dyslexic children

However even when vision defects may not be the cause of dyslexia the presence of

uncompensated vision defect in a dyslexic child may constitute difficulty while reading

and should be compensated for Improvement in vision will make reading easier for the

dyslexic child

64 The Study Strengths

The study methodology was based on reports found in other studies and consisted of the

following

a Classification The subjects selected for the dyslexic group represent a dyslexic population

that was classified according to the criteria used in the study by Latvala et al 3 and Evans et

139

al 100 The study was undertaken at a school for learning disabled learners and all students

were assessed and categorized as being dyslexic by the psychologists

b Study Examiner Examination of the subjects was conducted by the same optometrist who

had some experience in paediatric optometry This minimised the possibility of bias and

inter-examiner variability

c Examination Techniques The examination techniques used in this study were the same

as the techniques used in major referenced studies which were published in peer-reviewed

journals

d International Relevance It is the first study that has assessed broad vision functions in

a dyslexic population in an African setting

6 5 Limitations of the Study

The factors which may limit the generalization of findings of the study are as follows

a Inattentiveness This occurred in some children after numerous tests and the possibilities

of their lack of understanding of certain instructions However this problem was

minimized as trial readings were taken for some procedures and the participants were

instructed accordingly As stated in Section 34 testing was discontinued when the child

was tired This approach helped to minimize fatigue which could have negatively

affected the results In addition objective techniques that required minimal responses

from the participants were utilized in several procedures such as the cover test

retinoscopy and the monocular estimation method for evaluating accommodation posture

b Convenience Sampling This method of sampling was used given the limited number of

available subjects from the target population The available studies 235687-91102103 on

the subject of dyslexia learning disabilities and vision also used the convenience

sampling method

140

c Generalised Findings The generalization of the findings of this study is limited by the

fact that the refraction data was collected without the use of cycloplegia At the inception

of the data collection the diagnostic drug usage by optometrists was not approved in the

scope of practice in South Africa

d Sample Size The sample size was small as there was only one school for dyslexic

African children in Durban at the time of this study However review of the literature

also reveals that the sample sizes on studies conducted on dyslexia learning disabilities

and vision were typically small The targeted sample size of one hundred for this study

could not be met due to the limited number of subjects available The average sample size

of eight published studies 356 8790100 102 conducted on dyslexic children was 41

e Suppression Control The assessment of binocular accommodation facility and the

vergence reserves were performed without controlling for suppression

f Assessment of Relative Accommodation the use of a Phoropter would have yielded a

more conclusive result

g Eye Movements these were not assessed

Despite the acknowledged limitations inherent in this study the study provides useful

information and a research perspective on the prevalence of vision defects in a South African

population of dyslexic children which has not been conducted before

66 Recommendations

The following recommendations are made based on the findings of this study

661 Future Research

141

1 The same study with a similar protocol larger sample size with randomized sampling

2 Investigate the relative accommodation in in both groups using the phoropter

3 Assess the binocular functions with suppression control

4 Assess refractive error under cycloplgia

662 General Recommendations

1 All dyslexic children should have their eyes examined routinely to rule out

possibility of vision defects impacting on their reading performance

2 The Department of Education be reminded that visual defects can impact negatively on

the reading ability of learners and that teachers need to be more aware of pupils who may

present with signs of visual problems such as holding book too close or battling to see

the chalk board

3 The principals of the mainstream school should also be informed that vision defects were

detected in the non-dyslexic children

67 Conclusion

As a neurological condition which manifests primarily as a language-based disorder with

difficulty with words dyslexia requires a multi-disciplinary approach to its management with

an eye examination being needed to eliminate any possible effects that ocular conditions may

contribute to reading problems

The study provided the prevalence of disorders of visual acuity and refractive errors near point

of convergence accommodation dysfunction heterophoria strabismus and fusional reserves

among African dyslexic children of age range 10-15 years The only vision variable that was

more prevalent and that is significantly associated with dyslexia was the binocular

accommodation facility while only the near point of convergence (break and recovery) was

significantly more prevalent in the control than the dyslexic group The existence of these

statistically significant differences between the two groups may not imply clinical relevance

due to the small sample size The comparison of vision characteristics between the dyslexic

142

and control group indicates that the dyslexic children are not more at risk of having these

vision condition than the non-dyslexic children although sample size was small

Further research on African children will add to the body of knowledge about this group with

respect to the relationship between vision conditions and dyslexia about which very little is

known Only a few studies have provided evidence of the extent of dyslexia among African

children yet even a small percentage could result in many millions of children being affected

given the population of Africa While the problems that result from dyslexia are not life

threatening they do affect peoplersquos opportunities and quality of life

Unfortunately many African countries do not have the resources to provide specialized

schools for children with learning disabilities and every effort therefore needs to be made to

correctly identify the problems and possible causes of dyslexia In order for African countries

to implement the strategies to meet their millennium development education goals of ensuring

that by 2015 all children will be able to complete a full course of primary schooling they

need to provide not only for children in mainstream schools They need also to plan for

children whose academic ability would be enhanced by the appropriate diagnosis of learning

difficulties the provision of a simple eye test and spectacles or corrective apparatus where

appropriate

Dyslexia is not a disease for which a cure can be found rather long-term sustainable

interventions need to be put in place to ensure that the problems are identified and appropriate

remedial action is taken to minimize the impact it has on their life While many African

countries may not be able to afford schools for children with learning disabilities greater

awareness of the manifestation of the condition needs to be made to all teachers in an effort to

provide these children with the possibility of staying in main stream schools for as long as

possible where there are no alternatives With this in mind it is anticipated that this study will

contribute to the body of knowledge on vision conditions of African dyslexic school children

143

REFERENCES

1 Scheiman M Rouse M 1994 Optometric Management of Learning -Related Vision Problems Missouri Mosby Ist Edition

2 Ygge J Lennerstarnd G Axelsson I Rydberg A Visual Function in a Swedish Population of Dyslexic and Normally Reading Children Acta Ophtahlmol(Copenh)1993 71 (1) 1-9

3 Latvala ML Korhoenen TT Penttinen M Ophthalmic Findings in Dyslexic School Children Br J Ophthal 1994 78 339-343

4 Aasved H Ophthalmological Status of School Children with Dyslexia Eye 1987 161-68

5 Evans BJ Drasdo N Richards IL Investigation of some Sensory and Refractive Visual Factors in Dyslexia Vision Research 1994 34 (14) 1913-1926

6 Buzzelli AR Stereopsis Accommodative and Vergence Facility Do they relate to Dyslexia Optom Vis Sc 1991 68 (11) 842-846

7 Naidoo KS Africa Vision Research Institute Monograph 2007

8 Wu SY Nemesure B Leske MC Refractive Errors in a African Adult Population The Barbados Eye Study Invest Ophthal amp Vis Sci 1999 40 (10) 2179-2184

9 Tielsch JM Sommer A Katz J The Prevalance of Refractive errors Among Adults in the United States Western Europe and Australia Arch Ophthalmology 2004 122 495-506

10 Rudnicka AR Shahrul M Owen C Variations in Primary Open-Angle Glaucoma Prevalence by Age Gender and Race A Bayesian Meta-Analysis Invest Ophth Vis Sc 2006 47 (10) 4254 - 4265

11 Raju D 1997 Poor Oculomotor Control in Dyslexia Masters Degree Thesis University of Durban-Westville

12 Keily PM Crewther SG Crewther DP Is there an Association between Functional Vision and Learning Clin amp Exp Optom 2001 84 (6) 346-353

13 Krupska M Klein C 1995 Demystifying Dyslexia London Language and Literacy Unit

14 Schumacher J Hoffmann P Schma C Schulte G Genetics of Dyslexia The Evolving Landscape J Med Genet 2007 44 289ndash297

144

15 Pushpa S Nallur B Biological basis of Dyslexia A Maturing Perspective Current Science 2006 90 (2) 168-175

16 Sherman G International Dyslexia Association Fact Sheet 969 January 2000

17 Berninger V Dyslexia The Invisible Treatable Disorder Learning Disability Quarterly2000 23 (3) 125

18 Harrie RP Weller C Dyslexia httpwwwericecorg Last accessed January 2007

19 Critchley M Is Developmental Dyslexia the Expression of a Minor Cerebral Damage The William Copeland Memorial Lecture National Hospital London 1966

20 Christenson GN Griffin JR Wesson MD Optometrys Role in Learning Disabilities Resolving the Controversy J of Am Optom Assoc 1990 61 (5) 363-372

21 Evans B 2007 Dyslexia and Vision 2nd Edition Whirr Publishers London

22 Flax N Visual function in Dyslexia Am J Optom Arch Acad Optom 1968 45 574-87

23 Optometric Clinical Practice Guidelines Care of the Patient with Learning-Related Vision Problems America Optometric Association 2000

24 Tan L Spinks JA Eden GF Perfetti CA Siok WT Reading depends on Writing in Chinese Pro Natl Acad Sc 2005 102 (24) 8781 ndash 8785

25 httpfacultywashingtonedu What Is Dyslexia Last Accessed 15th September 2008

26 Shaywitz S Overcoming Dyslexia A new and complete science-based program for reading problems at any level 2003 New York NY Alfred A Knopf

27 Holland KC A look at the Reading Process Optician July 3rd 1987 11-13

28 Bedwell CH The Visual and Ocular aspects of Reading Difficulty Optician Sept 24th 1982 26-38

29 Rayner Keith Eye Movement in Reading Perceptual and Cognitive ProcessesCanadian Psychology Feb 1995

30 Sibylina M Brochure from Learning Disabilities Council of America 1998

31 Pavlidis GT Eye Movements The Diagnostic Key to Dyslexia S Afr OptomJuneAugust 1988 29-39

32 Evans B The Role of the Optometrist in Dyslexia Part 1 Specific Learning Difficulties Optometry Today 2004 January 30 29-33

iii

ACKNOWLEDGEMENTS

There are at times that this project seemed to be a lsquonever ending journeyrsquo and many people have encouraged me greatly I wish to express my profound gratitude and acknowledgement of the contributions of the following people who have given me support in different ways

Professor Kovin Naidoo (my supervisor) for his guidance throughout the entire research process

The examiners for their critical evaluation of the study

My family for their patience throughout the time I was not there for them Special thanks to my wife Joyce for all her encouragements right from the beginning of the project To my parents for giving me the discipline never to give up no matter how rough things may be

Carrin Martin for her invaluable support in editing this thesis Members of staff of the International Center for Eye Care Education Sandro Thaver for the statistical analysis Mirashnee Rajah and Diane Wallace for their continued administrative and moral support

All members of staff of the Optometry Department University of Kwazulu Natal for their assistance in different ways

Celia G of ILAMO for assistance with referenced articles

Department of Education for granting permission to have access to the schools

The Principals Parents and Learners of schools (Khulangolwazi special school and Addington Primary schools) Durban for their cooperation

Authors of the numerous studies I consulted whose studies upon which the foundation of this study was based

The University of KwaZulu-Natal for initial financial support

iv

Abstract

Dyslexia is a neurological disorder with genetic origin that affects a personrsquos word

processing ability their spelling writing comprehension and reading and results in poor

academic performance As a result optometrists are consulted for assistance with the

diagnosis and treatment of a possible vision condition Optometrists are able to assist with

treatment as part of a multidisciplinary management approach where optometric support is

necessary International studies have indicated that up to 20 of Caucasian school children

are affected by dyslexia while there are no similar figures for African children Studies have

been done to assess the extent of visual defects among Caucasian dyslexics but not among

African dyslexic children The aim of the study is therefore to determine the prevalence of

vision conditions in an African South African population of dyslexic school children and to

investigate the relationship between dyslexia and vision

The possible relationship between dyslexia and vision conditions has been recognized as an

important area of study resulting in research being conducted in many countries Studies

have been undertaken by optometrists and ophthalmologists who differ in their approach

and attitude on how vision conditions affect dyslexia A review of the literature revealed

three broad areas of vision that may impact on reading ability these being acuity defects

binocular vision and ocular pathology Acuity defects consist of visual acuity and refractive

error Areas of binocular vision evaluated in the literature include near point convergence

heterophoria strabismus accommodative functions vergence facility and reserves

Hyperopia was the only vision variable that was found to be consistently associated with

difficulties with reading but not causally while findings on other variables were

inconclusive However all the studies acknowledged the complexity of the condition and

the need for a comprehensive multidisciplinary management approach for its diagnosis and

management

The study was undertaken in the city of Durban South Africa using a case-control study of

two groups of African school children between the ages of 10 and 15 Both study groups

consisted of 31 children of normal intelligence who were matched in gender race and

v

socio-economic status The case group attended a school for children with learning

disabilities while the control group attended a mainstream school At the time of the study

only one school catered for African children with learning disabilities and only 31 of its

pupils were diagnosed with dyslexia Ethical approval was obtained from the University of

KwaZulu-Natal permission to undertake the study in the identified schools was obtained

from the Department of Education and the school principals consented on behalf of the

learners as it was not always possible to reach the individual parent

The researcher (an optometrist) visited both schools by appointment where rooms were

made available to do the testing and the tests were explained to all participants The

LogMar Acuity Charts were used to assess visual acuity and static retinoscopy was used to

assess refractive error Binocular vision was tested using the cover test for ocular alignment

the Hirschberg test for strabismus RAF rule for near point of convergence 2 D flipper

lenses for accommodation facilities Donderrsquos push up methods using the RAF rule for

amplitude of accommodation plus and minus lenses for relative accommodation monocular

estimation technique for accommodation posture and prism bars for vergence reserves

Ocular pathology was assessed using a direct ophthalmoscope

The dyslexic group presented with the following Refractive errors hyperopia 65

myopia 65 astigmatism 10 anisometropia 65 remote near point of convergence

33 esophoria at near 3 exophoria at near 95 accommodative infacility 54 and lag

of accommodation 3928 The dyslexic group had relatively reduced fusional reserve

compared to the control group

The control group presented with the following Refractive errors hyperopia 3

astigmatism 13 anisometropia 65 remote near point of convergence 48 esophoria at

near 0 exophoria at near 0 accommodative infacility 33 and lag of accommodation

4193

The prevalence of a remote NPC was higher in the control group than in the dyslexic group

and there was a statistically significant difference between the two groups NPC break

vi

(p=0049) and recovery (p=0046) The prevalence of poor binocular accommodation

facility at near was higher in the dyslexic group than in the control group and there was a

statistically significant difference between the two groups (p = 0027)

Vision defects such as hyperopia astigmatism accommodation lag convergence

insufficiency poor near point of convergence and accommodative infacility were present in

the dyslexic pupils but they were no more at risk of any particular vision condition than the

control group This study provided the prevalence of vision conditions in a population of

African dyslexic children in South Africa the only vision variable that was significantly

more prevalent in the dyslexic population being the binocular accommodation facility at

near although the study was unable to find a relationship between dyslexia and vision The

statistically significant difference may not imply clinical significance due to the small

sample size However it is recommended that any vision defects detected should be

appropriately compensated for as defective vision can make reading more difficult for the

dyslexic child

The sample size may have been a limitation however this was comparable with studies

reviewed most of which had sample sizes of less than 41 Due to the range of possible

ocular conditions that could affect dyslexia it is recommended that a larger sample size be

used to ensure more conclusive results Testing for relative accommodation with a

phoropter would provide more accurate results and accommodation facility and fusional

reserves would be better assessed with suppression control The study provides information

and an indication of research needs regarding the prevalence of vision defects in an African

South African population of dyslexic children

vii

TABLE OF CONTENTS

CONTENTS PAGETITLE PAGE iDECLARATION iiACKNOWLEDGEMENTS iiiABSTRACT ivTABLE OF CONTENTS viiLIST OF APPENDICES xiLIST OF TABLES xiiLIST OF FIGURES xiiiLIST OF PUBLICATIONS FROM THIS THESIS xiv

CHAPTER ONE INTRODUCTION 1

11 Background and Rationale for the Study 112 Aim 213 Objective 214 Research Questions 215 Significance of study 316 Goals of Study 317 Null Hypothesis 318 Research (alternative) Hypothesis 419 Scope of Study 4110 Assumptions 5111 Study Outline 5112 An overview of Dyslexia 5113 The Reading Process 8114 Eye Movement in reading 9115 Characteristics Features of Children with Dyslexia 10116 Implications of Dyslexia for learning 11

1161 Reading and Comprehension 121162 Handwriting 121163 Proof-reading 12

117 Prevalence of Dyslexia 12118 Aetiology of Dyslexia 13

1181 Neurological Factors 141182 Genetic Factors 151183 Peri-natal Factors 151184 Substance Use during Pregnancy 151185 Environmental Factors 161186 Nutrition 16

119 Diagnosis of Dyslexia 171191 Exclusionary Method 17

viii

1192 Indirect Method 171193 Direct Method 18

120 Functional Imaging Techniques used in the Diagnosis and Study of Dyslexia 191201 Positron Emission Tomgraphy (PET) 191202 Functional Magnet Resonance Imaging (fMRI) 19

121 Classification of Dyslexia 201211 Primary Dyslexia 211212 Secondary Dyslexia 21

122 Sub-types of Dyslexia 211221 Dysphonetic (auditoryphonetic) 211222 Dyseidetic (visual-spatial) 211223 Dysnemkinesia (motor) 211224 Dysphonetic-Dyseidetic 23

123 Theories of Dyslexia 231231 Phonological-Deficit Theory 231232 Cerebellar Theory 251233 Visual Deficit Theory 261234 Auditory Processing Theory 27

124 Cultural Differences in Dyslexia 28125 Treatment Intervention 30126 Role of Optometry in the Multidisciplinary Management of Dyslexia 30

1261 Case History 311262 Refractive and Binocluar Functions 311263 Visual-Motor Skills 311264 Visual-Spatial Analysis 321265 Short-term Visual Memory (visual-motor recall) 321266 Auditory Analysis Skills 32

127 Optometric Treatment Options 331271 Correction of Vision Defects 331272 The use of Coloured Overlay 33

128 Potential Mechanism for Benefit from Tinted Lenses 35129 Summary of Chapter One 37

CHAPTER TWO LITERATURE REVIEW 39

21 Introduction 3922 Visual Acuity 4023 Refractive Errors 47

231 Myopia 47232 Hyperopia 49233 Astigmatism 49234 Amblyopia 50235 Anisometropia 50

24 Near point of Convergence 53

ix

25 Heterophoria 5526 Strabismus 5827 Accommodation Functions 59

271 Amplitude of Accommodation 59272 Accommodation Facility 61273 Relative Accommodation 64274 Accommodation Posture 64

28 Vergence Facility 6529 Fusional Reserves 66210 Ocular Pathology 68211 Summary of Chapter Two 73

CHAPTER THREE METHODOLOGY 75

31 Introduction 7532 Research Design 75

321 Sampling Design 75322 Study population and Participants 75323 Inclusion Criteria for Dyslexic Group 76324 Exclusion Criteria for Dyslexic Group 76

33 Ethical Clearance 7734 Testing Protocol 7735 Study Setting 7736 Data Collection Technique 78

361 Visual Acuity 79362 Refractive Error 79363 Near Point of Convergence 80364 Strabismus 80365 Heterophoria 81366 Evaluation of Accommodation Functions 83367 Fusional Reserves 85368 Ocular Health 87369 Vergence Facility 87

37 Statistical Analysis 8738 Summary of Chapter Three 89

CHAPTER FOUR RESULTS 91

41 Introduction 9142 Prevalence of Vision Defects 93

421 Visual Acuity 91422 Refractive Errors 92423 Near Point of Convergence 94424 Ocular Health 96

x

425 Heterophoria 96426 Accommodation Functions 97427 Fusional Reserves 101

43 Summary of Chapter Four 105

CHAPTER FIVE DISCUSSION 107

51 Introduction 10752 Prevalence of Vision Defects 107

521 Visual Acuity 109522 Refractive Errors 116523 Heterophoria 116524 Near point of Convergence 118525 Accommodation Functions 120526 Fusional Reserves 126527 Ocular Pathology 130

53 Summary of Chapter Five 130

CHAPTER SIX CONCLUSION 132

61 Introduction 13262 Summary of Findings 134

621 Visual Acuity 134622 Refractive Errors 134623 Near Point of Convergence 135624 Heterophoria 135 625 Accommodation Functions 136 626 Fusional Reserves 137627 Ocular Pathology 137

63 Implications of the Study Findings 138 64 The Study Strengths 138 65 Limitations of the Study 139 66 Recommendations 140

661 Future Research 140662 General Recommendations 140

67 Conclusion 141

REFERENCES 143

xi

LIST OF APPENDICES PAGE

APPENDIX A Clarification of Terms 156APPENDIX B Request for Consent from Department of EducationAPPENDIX C Letter of Consent from Department of EducationAPPENDIX D Letter of Consent from Addington Primary schoolAPPENDIX E Visual Acuity Conversion Table

xii

LIST OF TABLES

TABLE TITLE PAGE

11 Characteristic Features of Dyslexia 1012 Features of the Magnocelular and Parvocellular systems 2721 Conclusions from Major Studies on Dyslexia and Vision 7031 Data Collection Tools and Measurement Outcomes 7832 The Diagnostic Criteria for each Test Variable 8741 Descriptive Statistics for Visual Acuity 9242 Descriptive Statistics for Refractive Error 9443 Descriptive Statistics for Near Point of Convergence Break

and Recovery Points 9544 Descriptive Statistics for Heterophoria 9745 Descriptive Statistics for Accommodative Posture 9846 Descriptive Statistics for Binocular Accommodation Facility 9947 Descriptive Statistics for Amplitude of Accommodation 10048 Descriptive Statistics for Relative Accommodation 10149 Descriptive Statistics for Base-in Vergence Reserves at Distance 102410 Descriptive Statistics for Base-in Vergence Reserves for Near 102411 Descriptive Statistics for Base-out Vergence Reserves at Distance 103412 Descriptive Statistics for Base-out Vergences at Near 103413 Descriptive Statistics for Dyslexic and Control Groups

for all Variables 104

xiii

LIST OF FIGURES

FIGURE TITLE PAGE

41 Prevalence of Visual Acuity 9242 Prevalence of Refractive Errors (total) 9343 Prevalence of the Types of Refractive Errors 9444 Prevalence of NPC Break 9645 Prevalence of Phoria at Near 9646 Prevalence of Accommodation Lag 9847 Prevalence of Reduced Binocular Accommodation Facility at Near 9948 Prevalence of Relative Accommodation 101

1

CHAPTER ONE INTRODUCTION

11 Background and Rationale for the Study

Optometry has long been involved in the subject of vision and learning As primary eye care

practitioners optometrists receive referrals from parents teachers psychologists and other

professionals who usually seek the optometristrsquos advice about whether a childrsquos vision problem

may be contributing to or is responsible for poor academic performance

According to Scheiman and Rouse1 learning which is the acquisition of knowledge or skill can

be achieved through study experience or instruction and that between 75 and 90 of what a

child learns is mediated through the visual pathways As vision plays a major role in reading and

the learning process any defects in the visual pathway disrupt the childrsquos learning

ldquoThe three interrelated areas of visual functions are 1

1 Visual pathway integrity including eye health visual acuity and refractive status

2 Visual efficiency including accommodation binocular vision and eye movements

3 Visual information processing including identification and discrimination

spatial awareness and integration with other senses

Several studies2-6 have been conducted to investigate the relationship between vision and dyslexia

on Caucasian populations while very few have been done on African subjects Consequently

most inferences and conclusions on dyslexia are based on research conducted in Caucasian

populations Naidoo7 reported that inferences drawn from studies in Caucasian populations may

not be appropriate for the African population as the lsquoAfrican Eyersquo differs from the Caucasian eye

For example epidemiological studies have reported variations in refractive errors where the

prevalence of myopia is higher among Caucasian persons than in African persons8 9 while

African populations had higher open angle glaucoma prevalence than Caucasian persons10

This study attempts to contribute to the literature by examining the prevalence of a broader

spectrum of vision variables in dyslexic school children in order to investigate a possible

2

relationship between dyslexia and vision in African school children in South Africa A search of

the literature revealed that very little research has been conducted in Africa the only one being

conducted in 1997 by Raju11 Raju11 investigated oculomotor functions in dyslexic school

children in South Africa and reported that 65 of the normal readers exhibited normal

oculomotor control as compared to 11 of the dyslexics Furthermore 54 of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills while 12 of the

controls displayed the same

12 Aim

The aim of the study was to determine the prevalence of vision conditions in a South African

population of African dyslexic children and to study the relationship between vision and dyslexia

13 Objectives

1 To determine the distribution of visual acuity disorders among African dyslexic children

2 To determine the distribution of refractive errors among dyslexic children

3 To determine the distribution of phorias among dyslexic children

4 To determine the distribution of strabismus among dyslexic children

5 To determine the distribution of accommodation disorders among dyslexic children

6 To determine the distribution of vergence disorders among dyslexic children

7 To determine the distribution of ocular pathology among dyslexic population

8 To compare these findings to a similar group of non-dyslexic children

14 Research Questions

1 What is the prevalence of vision conditions among African dyslexic children of age range

10 and 15 years

3

2 What is the relationship between dyslexia and vision

15 Significance of the Study

The study will attempt to

1 Identify high-risk ocular conditions among dyslexic children which might influence the

need for certain examinations or that could affect the childs ability to learn to read

2 Relate findings of this research work to that of previous researchers

3 Add to the knowledge base in the area of vision and dyslexia

16 Goals of Study

1 To obtain quantitative information on the prevalence of visual conditions in dyslexic

children in the African population

2 To obtain information on the relationship between vision disorders and dyslexia

3 To enable improvement of eye care for children with dyslexia

17 Null Hypothesis

1 There will be no statistically significant difference between the mean visual acuity of the

dyslexic group and of the control group

2 There will be no statistically significant difference between the mean refractive errors of the

dyslexic group and of the control group

3 There will be no statistically significant difference between the mean phoria in the dyslexic

group and of the control group

4 There will be no statistically significant difference between the strabismic mean angle of

deviation of the dyslexic group and of the control group

5 There will be no statistically significant difference between the mean accommodation

functions (amplitude facility posture) in the dyslexic group and of the control group

4

6 There will be no statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

1 8 Research (alternative) Hypothesis

1 There will be a statistically significant difference between the mean refractive errors in the

dyslexic group and of the control group

2 There will be a statistically significant relationship between the mean visual acuity of the

dyslexic group and of the control group

3 There will be a statistically significant difference between the mean of phoria of the

dyslexic group and of the control group

4 There will be a statistically significant difference between the strabismus mean angle of

deviation of the dyslexic group and of the control group

5 There will be a statistically significant difference between the mean accommodation

functions of the dyslexic group and of the control group

6 There will be a statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

19 Scope of Study

1 This study is limited to the assessment of visual functions in dyslexic and normal readers

The visual functions investigated include visual acuity refractive error ocular alignment

near point of convergence accommodation functions and vergence reserves The

assessment of eye movement was not part of the study

2 Screening for and diagnosing dyslexia was not part of this study The school

psychologistsrsquo reports were relied on in selecting subjects who qualify to participate in this

study The information was obtained from the learnersrsquo school files 3 The scope of vision parameters investigated is limited only to the prevalence and

relationship between these variables in dyslexic and control groups as outlined in the

hypothesis Correlation analysis of different variables was not part of this study

5

110 Assumptions

In this report dyslexia is used synonymously with ldquodevelopmental dyslexiardquo ldquoreading

disabilityrdquo and reading disorderrdquo and will be used to describe children who find it extremely

difficult to learn to read despite having normal intelligence appropriate educational opportunities

and absence of emotional disorders Children so defined have reading age that is two or more

years behind their chronological age12

111 Study Outline

This thesis is divided into six chapters Chapter one indicates the rationale of the study provides

an overview of dyslexia and details its purpose objectives significance and goals Chapter Two

(the literature review) gives a detailed review of literature in the area of dyslexia learning

disabilities and vision with particular emphasis to vision variables as outlined in the hypothesis

to be tested This is followed by methodology in Chapter Three which details the research

methodology in which a case-control study was done in two schools in Durban The areas of

vision variables evaluated include visual acuity and refraction binocular vision and ocular health

evaluation Chapter Four (Results) presents an analysis of the results obtained from the study

Chapter Five (Discussion) discusses the study findings and their meaning Chapter Six presents

conclusions from the study based on the study results indicating the implications applications

and recommendations for future study and the study limitations

112 An overview of dyslexia

Some children find it difficult to learn to read despite having normal intelligence appropriate

educational opportunities and absence of emotional disorders Their reading age is two or more

years behind their chronological age Such children are referred to as being dyslexics Where

there is an associated deficit in intelligence the condition may be described as generalized

learning disability12

6

The term dyslexia is derived from the Greek words dys meaning hard or difficult and lexia

from the word lexicos which means pertaining to words so dyslexia means difficulty with

words-either seen heard spoken or felt as in writing13

The World Federation of Neurologists13 and the International Classification of Diseases-1014

defined dyslexia as ldquoa disorder manifested by difficulty in learning to read despite conventional

instruction adequate intelligence and socio-cultural opportunity It is dependent upon

fundamental cognitive disabilities which are frequently constitutional in originrdquo

Dyslexia is a complex disability which affects different aspects of reading The acquisition of

reading related skills are coordinated by visual motor cognitive and language areas of the brain

Consequently dyslexia can result from deviation of normal anatomy and function of cognitive

and language areas in the brain 15

Dyslexia refers to a cluster of symptoms resulting in people having difficulties in specific

language skills particularly reading15 The mystery in dyslexia as a syndrome lies in the fact that

dyslexics have ldquodifficulty to learn to read despite having average or above average intelligence

adequate auditory and visual acuity absence of frank brain damage no primary emotional

problems and have adequate educational instructionsrdquo 16

Dyslexia is a developmental disorder that shows in different ways at different developmental

stages As documented by Krupska and Klein 13 dyslexia is a learning disability characterised by

problems in expressive or receptive oral or written language that manifest initially as difficulty in

learning letters and letter sound association which is followed by difficulty in learning letters and

reading words accurately which consequently results in impaired reading rate and written

expression skills (handwriting spelling and compositional fluency) Some dyslexics find it

difficult to express themselves clearly or to fully understand what others mean when they speak

The effects of dyslexia reach well beyond the classroom as it can affect the persons self-image

It makes them feel dumb13 17 Dyslexia describes a different kind of mind often gifted and

productive that learns differently An unexpected gap exists between learning aptitude and

achievement in school 18

Many dyslexics are creative and have unusual talent in areas such as art athletics architecture

7

graphics electronics mechanics drama music or engineering 18 Some dyslexic learn best with

a multi-sensory delivery of language content which employ all pathways of learning at the same

time seeing hearing touching writing and speaking18 It is primarily not a problem of

defective vision although underlying vision deficit has been known to exaggerate the dyslexics

reading problems 18

According to Critchley19 ldquothe child otherwise normal as regards to intellectual status and

emotional stability finds an isolated difficulty in mastering the significance which lies behind

printed or written symbols and of necessity finds it extremely hard to align the verbal

components of speech (as emitted by the mouth and as received by the ear) with the empirical

characters which are set out on paper in one form or anotherrdquo Dyslexics are unable to acquire

written language skills through ordinary learning and teaching methods and often fail to progress

in education19

A dyslexic individual displays a characteristic pattern of decoding (recognising) and encoding

(spelling) difficulties with written words20 Interestingly dyslexics may not have overt

difficulties with spoken language yet do have marked difficulties with written languages21 The

difference between written and spoken language is that written language is seen while spoken

language is heard This fact may explain why vision has always been implicated on the topic of

dyslexia 21

Dyslexics experience difficulties with visual-verbal matching necessary to establish word

recognition ability Even when a clear single visual image is present the dyslexic may be unable

to decode the printed word in some cases This may be related to the fact that the difficulty that

dyslexics have is at a much more fundamental stage in the reading process so the role of vision

function is quite different in dyslexia than in other types of reading difficulty22 (which may be

peripheral such as hyperopia leading to difficulty in reading) The two main situations in which

the term dyslexia now commonly applies are when the reader has difficulty decoding words (that

is single word identification) and the second is encoding words (spelling) The second type is a

frequent presentation in optometric practice that is when the reader makes a significant number of

letter reversals errors (example b-d) letter transposition in word when reading or writing

(example sign for sing) or has left-right confusion 23

8

113 The Reading Process

Language development entails four fundamental and interactive abilities listening speaking

reading and writing while learning to read involves a complex system of skills relevant to visual

(the appearance of a word) orthographic (visual word form) phonological and semantic

(meaning of word and phrases) processing and a variety of behaviors such as letter naming letter

perception word recognition and comprehension each of which uses a different part of the brain

Reading is an interpretation of graphic symbols24-26

For some children the ability to break a word into its smaller parts a task crucial in reading is

extremely difficult Dyslexia is a localized problem one involving the sounds and not the

meaning of spoken language Speaking is natural while reading is different it must be learned

To be able to transform the printed letters on the page to words that have meaning the letters

must be connected to something that already has inherent meaning- the sounds of spoken

language In order to learn to read children must learn how to link the printed letters on the page

to the sounds of spoken language and understand that words are made up of sounds and that

letters represent these sounds or phonemes24- 26 In other words learning to read requires that the

child be able to break the word into the individual components of language represented by letters

and to be able to tell the difference between the individual phonemes that make up a word

Learning to read requires that the central principle behind the alphabet is understood that is

words are made of phonemes and in print phonemes are represented by letters Phonemes are

the shortest units of sound that can be uttered in a given language and that can be recognized as

being distinct from other sounds in the language For example the word cat has three phonemes

ca ah and ta Blended together the sounds form the word rsquocatrsquo24-26 When one speaks the

sounds are blended ttogether and are said one cat 24-26 To learn to read a child must learn that

there are three separate sounds This is difficult for children with dyslexia The inability to break

word into its parts is the main reason why children with dyslexia have trouble learning to

read24- 26

Furthermore dyslexic children find it difficult to bring the print to language (such as when asked

to read what they just wrote) For example a child can copy the letters w-a-srdquo correctly but

9

when asked what was written a child with dyslexia may reply saw The problem in this case

may not be related to vision but rather one of perceptual skills of what the child does with a word

on a page The brain mechanism of going from print to language is phonologically based 24-26

Reading acquisition builds on the childrsquos spoken language which is already well developed

before the start of formal schooling Once a written word is decoded phonologically its meaning

will become accessible via the existing phonology-to-semantics link in the oral language system

Thus the childrsquos awareness of the phonological structure of sound of speech plays a major role in

the development of reading ability 24-25

114 Eye Movement in Reading

For a person to be able to read a letter or word the eye has to first identify the written material

Normally the eyes scan across the line of words on the page stopping to fixate at various points

then making short jumps from one position to the next (some saccade eye movements are

involved) Typically there are around five fixation pauses in a line Sometimes the eye will

regress and go back to a previously passed over word element A reader views the text mainly

with the controlling or reference eye whilst the other eye diverges and converges within fine

elements27 A sophisticated mechanism27 (a connection of the visual receptors of the retina with

the visual nervous pathway to the brain) 28 ensures control of this drift and prevents diplopia from

being consciously observed These drifts and subsequent recovery can be clearly seen by a

number of techniques including infrared observation of the eyes27

The minimum time a person can spend on fixation and move to the next is around 250

milliseconds27 For skilled readers the average saccade length is 7-9 letter spaces and the

duration of saccades is of the order of 25-50 milliseconds29 while the average fixation pauses last

between 200-250 milliseconds27 29 Thus our eyes are still for some 90 of the time while

reading and are subject to large changes in acceleration and deceleration Even a good reader

will regress and go back 10 to 20 of the time27 29

The textual image seen by the eyes is imaged onto both maculae and transmitted to both cerebral

10

cortex where it is first received in the primary visual cortex (Brodmanrsquos area 17) with some

initial processing of colour and tone occur Progressively higher levels of processing occur in

adjacent areas 18 and 19 with the highest level of visual language processing taking place in the

angular gyrus (area 39) The angular gyrus is central to reading since it is here that hearing

speech and vision meets and the written word is perceived in its auditory form The auditory

form of the word is then processed for comprehension in Wernickersquos area as if the word had been

heard25 27

115 Characteristic SignsSymptoms of Children with Dyslexia

Dyslexia is heterogeneous in nature and there is no single pattern of difficulty that affects all

people Dyslexic children frequently show a combination of characteristics (Table 11)

Table 11 Characteristic SignsSymptoms of Dyslexia 16 30-31

Reading Visionminus Holds book too close

minus Word reversals

minus Skips complete words

minus Re-reads lines

minus Points to words

minus Word substitution

minus Sees double

minus Poor comprehension in oral reading

minus Might see text appearing to jump around on a page

minus Unable to tell difference between letters that look similar in shape such as o and e

minus Unable to tell difference between letters with similar shape but different

orientation such as b and p and q

minus Letters might look jumped up and out of order

minus Letters of some words might appear backwards eg bird looking like drib

11

Spelling

minus Omission of beginning or ending letters

minus Can spell better orally than written

minus Letter reversals

minus Wrong number of letters in words

Difficulty with Reading

minus Difficulty learning to read

minus Difficulty identifying or generating rhyming words or counting syllables in

words (Phonological awareness)

- Difficulty with manipulating sounds in words

minus Difficulty distinguishing different sounds in words (Auditory discrimination)

AuditoryVerbalminus Faulty pronunciation

minus Complains of ear problems

minus Unnatural pitch of voice

minus Difficulty acquiring vocabulary

minus Difficulty following directions

minus Confusion with beforeafter rightleft

minus Difficulty with word retrieval

minus Difficulty understanding concepts and relationships of words and sentences

116 Implications of Dyslexia for Learning

Dyslexia affects people in different ways and depends on the severity of the condition as well as

the effectiveness of instruction or remediation The core difficulty is with word recognition and

reading fluency spelling and writing Some dyslexics manage to learn early reading and spelling

tasks especially with excellent instruction but later experience their most debilitating problems

when more complex language skills are required such as grammar understanding textbook

material and writing essays The dyslexic syndrome therefore affects reading and learning in the

12

following ways263034

1161 Reading and Comprehension

One major difficulty dyslexics have is the time it takes to read and understand a text This makes

reading an extremely difficult task and they may report that they get tired and may get headaches

after reading for a short while30 There is also difficulty in assimilating what they have read even

when they know the words Multiple-choice questions are usually difficult for them as they find

it difficult to recognise words out of context263034

1162 Handwriting

Dyslexics have problems with handwriting since for some forming letters takes concentration

that their ideas get lost and their written expression suffers30

1163 Proof-reading

Dyslexics with visual processing difficulties usually have difficulty in identifying errors in

their own writings Reading difficulties make it uneasy for them to see their own errors in

expression and sentence structure grammar26 30

117 Prevalence of Dyslexia

Estimates of the prevalence of dyslexia in Caucasian populations vary from 5 to 20 in the

general population16 Pavlidis 31 reported that about 1 to 3 of the total population of children

suffers from dyslexia and that 20 to 30 of those cases classified as ldquogeneral reading failures

are probably dyslexics Evans32 reported a prevalence of 4 - 5 in children and stated probably

more boys than girls are dyslexic Park33 documented that up to 15 of the school-age

population has some degree of dyslexia with a ratio of four boys to one girl The large variability

in the prevalence reported by different authorities may be attributed to the differences in the

diagnostic criteria and the cut-off point applied to the psychometric tests14 Dyslexia is the most

common and the most researched type of learning disability34

13

A literature search revealed one study in Egypt that reported the prevalence of specific reading

difficulty in 2nd and 3rd grades in elementary school population to be 1 and the male to female

ratio to be 27 to 1 The authors concluded that the prevalence was low compared to that reported

in western countries and that the difference may be due to the way the Arab language is written

and read 35

Contrary to the above reports that males tend to be more dyslexic than females Guerin Griffin

Gottfried and Christenson 36 studied dyslexia subtypes and severity levels with respect to gender

differences The authors36 concluded that

when objective criteria such as the discrepancy between achievement (in

reading andor spelling) and intelligence or direct assessment of coding

skills are used as criterion and non-referred or random samples are

assessed the ratio of males to females identified as dyslexics approaches

11 When identification depends on referrals from parents and or

teachers more males than females may be detected because of the nature

and intensity of boysrsquo behavioural reactions to the disability

According to Shaywitz and Shaywitz34 good evidence based on sample surveys of randomly

selected population of children now indicates that dyslexia affects boys and girls comparably

118 Aetiology of dyslexia

The literature contains diverse theories (not less than four major theories) on the aetiology of

dyslexia probably due to the complex nature of the subject but several authors13-15 26 37-42 have

maintained that it is a neurological disorder with a genetic origin Despite decades of

multidisciplinary research neuropsychology brain anatomy and neuro-imaging 37-42 the specific

causes of dyslexia are still unknown The leading assumption however is that dyslexia

fundamentally stems from subtle disturbances in the brain and other possible explanations of the

cause of dyslexia such as language processing difficulties are based on the neurological etiology

14

The aetiological considerations in dyslexia are

1181 Neurological Factors

As documented by Habib 37 the concept of the neurological basis of dyslexia was first mentioned

independently by a Scottish ophthalmologist Hinshelwood in 1895 and a British physician

Morgan in 1896 Both found similarities of certain symptoms of dyslexia in some children

Dyslexia was then described as ldquovisual word blindnessrdquo The earlier understandings for the

neurological basis of dyslexia came from neuro-pathological studies of brains from dyslexic

persons According to Habib37 it was first reported by the French neurologist Dejerine in

1891 that damage to the left inferior parieto-occipital region (in adults) resulted in variable

degree of impairments in reading and writing suggesting that the left angular gyrus may play a

role in processing the ldquooptic images of letterrdquo It was subsequently thought that impaired reading

and writing in the young dyslexic patients could be due to abnormal development of the same

parietal region which was damaged in adult alexic patients 37

Another perspective 13 37 is that the dense distributions of ectopias (the presence of neural

elements in places where they are not supposed to be found) all over the cerebral cortex

(particularly in the perisylvian language areas) resulted in alteration of brain organisation One

such alteration in dyslexia is the lack of asymmetry in a language-related cortical region called

the planum temporale (an auditory area that lies on the superior surface of the temporal lobe) In

normal subjects the platinum temporale is usually larger in the left hemisphere However the

dyslexics showed symmetry of the planum temporale 13 37 Furthermore these ectopias may

reduce cortical connectivity as suggested by recent positron emission tomography and magnetic

resonance imaging studies 37

Another speculation was that the development of ectopias could be due to foetal hormonal

(possibly testosterone) imbalances during late pregnancy which could also account for the

possible male predominance in dyslexia 15 30 37 38

The maturation lag which is believed to be due to the slower development of the nerve fibres of

the corpus collosum (bundle of cells connecting the left and right hemisphere of the brain) has

also been theorized to be a cause of dyslexia A delay in the growth of the fibres linking the two

15

hemispheres would result in affected children being physiologically incapable of tasks which

require hemispheric organisation This leads to the inadequate development of the language

functions 12 37 However recent neuro-imaging studies 39-42 have shown that impaired reading of

alphabetic scripts (such as in the English language) is associated with dysfunction of the left

hemisphere posterior reading systems primarily in the left temporo-parietal brain regions

1182 Genetic Factors

Genetic considerations in dyslexia are derived from the fact that dyslexia runs most often in

families However it is speculated that different forms of dyslexia may occur within the same

family which also may means that it seems likely that the inheritance is indirect 15 31 However

genetic markers have been identified in chromosome 18 38 and chromosomes 6 and 15 15 In

fact chromosomes 2 (DYX3) 6 (DYX2) and 15 (DYX1) in dyslexics have been reported to be

inherited in an autosomal dominant mode of transmission 15 Furthermore Ramus 43 stated that

ldquogenetically driven focal cortical abnormalities such as ectopias and microgyri in specific areas

of left perisylvian cortex involved in phonological representations and processing are the

primary cause of dyslexia which is consistent with anatomical studies of dyslexic brains showing

loci of cortical abnormalities functional brain imaging studies showing that the very same areas

are involved in phonological processing and show abnormal activation in dyslexicshelliprdquo

1183 Peri-natal Factors

Peri-natal factors may manifest as problems during pregnancy or complications after delivery

More importantly it can also happen that the mothers immune system reacts to the foetus and

attacks it as if it was an infection It has also been theorized (based on partial evidence) that

dyslexia may occur more often in families who suffered from various autoimmune diseases (such

as rheumatoid arthritis Gravesrsquo diseases multiple sclerosis systemic lupus Sjogrenrsquos

syndrome)3137

1184 Substance Use during Pregnancy

16

Drugs such as cocaine (especially in its smokable form known as crack) seem to affect the

normal development of brain receptors of the foetus It has been documented that mothers who

abuse substance during pregnancy are prone to having smaller babies and such infants are

predisposed to a variety of problems including learning disorders Thirdly alcohol use during

pregnancy may influence the childrsquos development and lead to problems with learning attention

memory or problem-solving There is also a risk of foetal alcohol syndrome (characterised by low

birth weight intellectual impairment hyperactivity and certain physical defects) 30

1185 Environmental Factors

Environmental factors are risk factors for the development of reading disabilities 44 Reading

disabilities which are due to environmental influences have been referred to as non-specific

reading disabilities 20 or pseudo- dyslexia as the reading disorder is secondary to extraneous

factors and genetic factors play an important role 45 ldquoEnvironment factors play a significant role

in the level to which neurological impairments manifest themselves Encouraging nurturing and

stimulating surroundings can reduce the impact of risk factors on learning performance and life

skillsrdquo 44 Environmental circumstances which may profoundly influence the dyslexic child

include parental attitude the role of multilingualism in the home attention problems the

drawback of frequent changes of school the personality of the teacher and the childs emotional

reaction to his difficulties 152030

It seems that the influence of environmental factors may best be described as a potential risk

factor than a causative factor in dyslexia as there are diverse views on the relationship between

environmental influences and dyslexia According to Grigerenko46 socioeconomic level

educational opportunity and home literacy level are major environmental risks for the

manifestations of developmental dyslexia hellip the ldquomagnitude of the effects was not strong enough

to view them as powerful causal factorsrdquo

1186 Nutrition

Nutrition plays an important role in bridging the gap between genetic constitution and a childs

potential for optimal development Nutrition is a basic requirement for the maintenance of

17

growth and overall development while malnutrition and under-nutrition are associated with

disorders in the normal development of intelligence perceptual maturation and academic

achievements 47 Studies conducted by Grant Howard Davies and Chasty 48 showed that there

is an association between dyslexia and low concentration of zinc Zinc is necessary for normal

brain development and function and essential for neurotransmission44

119 Diagnosis of Dyslexia

The first stage in the diagnosis of dyslexia is with awareness by parents or educators that a

problem in reading exists The initial point of assessment starts with the medical practitioner

(possibly a paediatrician) who conducts a complete medical examination and obtains a

comprehensive health history A correct diagnosis of dyslexia starts with a very well taken case

history especially good awareness of the childrsquos developmental history This may give

indications of any medical condition that may be contributing to reading difficulties If indicated

the child may be referred for a neurological examination If dyslexia is suspected the physician

may then refer the child for further evaluation and treatment by a specialist in psycho-educational

assessments In most cases the diagnosis of dyslexia is usually performed by the psychologist

The Psychologists conduct psychometric assessments which include assessment of intelligence

quotient (IQ) various aspects of reading performance spelling mathematics sequential memory

and other aspects of cognitive and emotional development49 50 The major purpose of the

diagnostic process is to isolate the specific difficulties associated with dyslexia and to suggest

appropriate educational intervention 49 50

There are three methods for diagnosing dyslexia

1191 Exclusionary Method

In the exclusionary method the diagnosis of dyslexia is made when all factors involved in a

nonspecific reading disability (reading problems resulting from factors such as intellectual

disability dysfunction of hearing or vision inadequate learning experience socio-cultural

deprivation primary emotional problems poor motivation) are excluded 2051 The problem with

18

the exclusionary method of diagnosing dyslexia is that dyslexia cannot be diagnosed until after

the child has been failing in school for almost two years By this time constant failure may have

produced a negative attitude towards school and psychological problems 31

It provides a very

limited description of the features of the disorder Dyslexia is often defined in terms of the

discrepancy between reading achievement and chronological age or grade level failing to

consider the possible overlap between some of the intelligence quotient tests and reading tests 52

According to Snowling 53 IQ is not strongly related to reading and so the use of IQ in defining

dyslexia is inappropriate stating that many children with low IQ can read perfectly well and that

the discrepancy definition (based on the difference between reading achievement and

chronological age) of dyslexia cannot be used to identify younger children who are too young to

show a discrepancy

1192 Indirect Method

The second method of diagnosing dyslexia is the indirect approach A diagnosis is sought by

attempting to associate neurological soft signs such as finger ldquoagnosiardquo with reading failure The

drawback with this method is that many dyslexics do not manifest neurological soft signs 20

Another type of the indirect method involves the analysis of cognitive tests An example is the

Wechsler Intelligence Scale for Children (Revised) (WISC-R) WISC-R which compares ldquoverbalrdquo

versus ldquoperformancerdquo 20

1193 Direct Method

The third method which seems most reliable is the direct method which involves the use of

characteristic decoding and spelling patterns to determine the specific dyslexia It has the

additional advantage of addressing the fact that dyslexia is heterogeneous 20

19

120 Functional Imaging Techniques used in the Diagnosis and Study of

Dyslexia

There are functional imaging techniques for diagnosing and studying brain activity in dyslexia

These techniques include positron emission tomography (PET) Magnetic resonance imaging

(MRI) and computed tomography (CT) The two major techniques include

1201 Positron Emission Tomography (PET)

PET is a non-invasive imaging technique that uses radioactively labeled compounds to

quantitatively measure metabolic biochemical and functional activity in living tissue It

assesses changes in the function circulation and metabolism of body organs 54 55 The PET is

a sophisticated nuclear medicine technique that looks directly at cerebral (brain) blood flow

and indirectly at brain activity (or metabolism) The most commonly used labels are

flourophores which emit light when stimulated with light of the appropriate wavelength and

radio nuclides which emit gamma rays or beta particles when they decay56 The dose of

radionuclide injected is minute and does not pose any significant hazard Various isotopes are

used depending upon the brain region and function studied54

Unlike MRI or CT imaging which primarily provide images of organ anatomy PET measures

chemical changes that occur before visible signs of disease are present on CT and MRI images 54 While PET is most commonly used in the fields of neurology oncology and cardiology

applications in other fields are currently being studied 57

1202 Functional Magnetic Resonance Imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a noninvasive usually painless procedure that

uses a powerful magnetic field radio waves and a computer to produce detailed pictures of

organs soft tissues bone and virtually all other internal body structures The images can then be

examined on a computer monitor or printed The fMRI does not use ionizing radiation57 58

Functional magnetic resonance imaging (fMRI) is a relatively new procedure that uses MR

imaging to provide a picture of the brains active rather than its static structure The fMRI

20

machines measure the level of oxygen in the blood through a technique called blood level-

dependent (BOLD) contrast The two different states of hemoglobin-oxygen-rich oxyhemoglobin

and oxygen-poor deoxyhemoglobin differ in their magnetic properties The huge fMRI magnets

are sensitive to changes in the concentration of deoxyhemoglobin As neural activity increases

blood flow to the vasculature of the brain presumably increases altering this concentration The

more active a brain area is the more blood flows to it Thus fMRI can provide a moment-by-

moment movie of brain activity

Studies using fMRI are beginning to shed light on dyslexia Functional magnetic resonance

imaging (fMRI) is similar to MRI as both procedures use a similar scanner The difference is that

the fMRI uses somewhat more sophisticated hardware and software that allow it to capture brain

changes (mainly blood flow) as a person performs a specific cognitive task The functional

magnetic resonance imaging (fMRI) scanner measures where blood flows through the brain

during certain tasks26

Earlier studies of dyslexia were based on histo-pathological studies According to Habib37 the

fMRI has an added advantage that cortical anatomy of the dyslexic brain can be demonstrated

thus answering the criticism addressed to results from pathological findings

120 Classifications of Dyslexia

Dyslexia can be classified into two broad categories according to its presumed aetiology The

following methods of classification have been documented by Helveston59

1 Primary dyslexia (specific developmental dyslexia)

2 Secondary dyslexia

1211 Primary Dyslexia

This is considered to be caused by a specific central nervous system defect This neural defect is

thought to be located in the angular gyrus of the dominant hemisphere It is often hereditary and

21

may affect more boys than girls In primary dyslexia the decoding process involved in language

system is affected and reading becomes extremely difficult for the child However intellectual

tasks that do not involve reading text are unaffected 59

1212 Secondary Dyslexia

There are two types of secondary dyslexia59 Endogenous and Exogenous

a Endogenous Dyslexia this results from pathological changes in the central nervous

system secondary to trauma or disease such as childhood meningitis hydrocephalus

with brain damage and porencephalus59 Other medical causes include prematurity

congenital hydrocephalus encephalitis traumatic brain injury and lead or

methylmercury poisoning50

b Exogenous Dyslexia this results from intellectual deprivation usually limited to lack of

adequate educational opportunity or motivation59 or due to low intelligence socio-

cultural deprivation primary emotional problems sensory impairment (visual or

auditory) poor motivation or attention problems20 Reading disability due to these

mentioned factors have been referred to as non-specific reading disability20

1 22 Sub-types of Dyslexia

When children first start to read they learn to recognise simple words by their shapes (sight

analysis) building their sight vocabulary This is followed by learning how to analyse

complicated words by breaking them down into their sound components (phonetic analysis)

The dynamics of the two processes (Sight-analysis and Phonetic analysis) are important in

explaining how words are decoded based on the neurologic-behavioural model documented by

Christenson et al 20 and Evans 21 Dyslexia can be divided into the following subtypes 20 21

22

1221 Dysphonetic (auditoryphonetic)

The individual has a minimal dysfunction involving the Wernickes area Readers have an

extremely difficult time developing phonic word analysis or decoding skills Their typical

reading errors are wild guesses (lsquolikersquo for lsquolittlersquo) errors picking words with similar shapes

(horse for house ) and semantic substitutions (home for house) Their typical spelling errors

are phonetically inaccurate (reffor rough) This type of reader persists in using whole-word

approach and will guess at unfamiliar words rather than using their word analysis skills While

they do not often continue to reverse letters and words after the age of eight they often make

spelling reversals (For example interver for inventor and wirters for writers) Typically

they often look at the first and last letter and the length of the word and then guess (monkey for

money or stop for step)20 21

1222 Dyseidetic (visual-spatial)

In dyseidetic type of dyslexia dysfunction occurs in the angular gyrus The individual have high

ability in phonic word analysis They often have a difficult time learning the letters of the

alphabet They usually have a very low sight vocabulary poor visual memory poor perceptual

skills and letter and word reversals (dig for big or saw for was) Their reversals often last

past the age of eight and include both reading and writing reversals This type of child is often a

very slow reader who often will try to sound out familiar as well as unfamiliar words They spell

the word as it sounds By first identifying the learning style of these children probably during the

case history the optometrist may know the childs strengths and weaknesses Vision therapy can

then be designed to fit each childs needs Enhancing the childrsquos visual perceptual and eye

tracking skills plus having the schools teach these children according to their strengths enable

them to achieve their full potential in school 20 21

122 3 Dysnemkinesia (motor)

Dysnemkinesia is a less serious form of dyslexia and it involves minimal dysfunction of the area

of the motor cortex involved in letter formation Dysnemkinesics can be distinguished by their

23

frequent letter reversals such as d for b as in doy for boy Additional dyslexic types occur

when combinations of the three basic types arise such as dysphoneidesia dysnemkineidesia

dysnemkinphonsia and dysnemkinphoneidesia20 21

1224 Dysphonetic-Dyseidetic

Dysphonetic- dyseidetic reader has a weakness in both whole-word and phonic analysis Their

reading errors include wild guesses (fish for father) and word reversals (no for on) Their

spelling is usually phonetically inaccurate and often bizarre (rit for faster) 2021

123 Theories of Dyslexia

Assisted by the increasing body of knowledge obtained by neurophysiological and imaging

studies14 several theories have been proposed with the aim of characterizing the fundamental

processes underlying dyslexia14 These theories can be classified under four major frameworks

phonological visual cerebellar and auditory

1231 PhonologicalndashDeficit Theory

According to the phonological theory affected individuals have difficulties in perceiving and

segmenting phenomes leading to difficulties in establishing a connection between phonemes and

graphemes 14 The brain recognizes language in a hierarchical order The upper levels deal with

semantics (the meaning of words) syntax (grammatical structure) and discourse (connected

sentences) The lower levels of hierarchy deal with breaking sounds into separate small units

called phonemes (sound units) Thus before words can be comprehended at higher levels in the

hierarchy it has to be broken down into phonologic constituents that the alphabetic characters

represents15 26 An adequate reading development stems from some considerable level of spoken

language already acquired by a child in the early years of life Consequently failure to develop

the association between letters (grapheme) and sound (phoneme) has been considered to be a

major cause of reading and spelling impairment in most cases of developmental dyslexia60-62

24

Reading requires some skills of phonological processing to convert a written image into the

sounds of spoken language1 According to the phonologic-deficit hypothesis people with

dyslexia have difficulty developing an awareness that words both written and spoken can be

broken down into smaller units of sound and that in fact the letters constituting the printed word

represent the sounds heard in the spoken word2

According to Habib37 neuropsychological studies have provided considerable evidence

indicating that the main mechanism leading to reading difficulties is phonological in nature and

that it has been repeatedly demonstrated that the core deficit responsible for impaired learning to

read is phonological in nature and has to do with oral language rather than visual perception At

the brain level this cognitive deficit would arise from a congenital dysfunction of certain cortical

areas involved in phonology and reading60 At the neurological level it is usually assumed that

the origin of the disorder is a congenital dysfunction of the left-hemisphere perisylvian brain

areas underlying phonological representations or connecting between phonological and

orthographic representations60

Brain imaging studies in dyslexics in response to a phonological task indicate under-activation of

posterior brain regions (Wernickesrsquos area angular gyrus and striate cortex) and relative over-

activation in anterior regions (inferior frontal gyrus) These brain activation patterns provide

evidence of an imperfectly functioning brain system for segmenting words into their phonologic

constituents39 42 Since most dyslexics show deficits in phoneme processing the phonological

deficit is the most significant consistent marker of dyslexia and remains the predominant theory15343739 hence the focus on phonology in the majority of remedial programmes 63

Based on the phonological deficit theory the other symptoms of dyslexia are considered as

simple co-morbid markers that do not have a causal relationship with reading disability62

However proponents of the phonological theory typically contend that these disorders are not

part of the core features of dyslexia62 A major setback with the phonological theory is that it

does not account for symptoms such as impairment in visual perception and problems with motor

coordination that are found in dyslexics 15 and does not account for symptoms of dyslexia that are

unrelated to phonetic decoding difficulties such as problems with short term memory and visual

25

processing issues64

The two basic processes involved in reading are decoding and comprehension In dyslexia a

deficit at the level of the phonologic units impairs the ability to segment the written word into its

underlying phonologic elements Consequently the reader experiences difficulty initially in

decoding the word and then in identifying it The phonologic deficit is independent of other non-

phonologic abilities especially the higher-order cognitive and linguistic functions involved in

comprehension such as general intelligence and reasoning vocabulary and syntax are generally

intact A deficit in phonologic analysis and an ironical intact higher-order cognitive ability offers

an explanation for the paradox of otherwise intelligent people who experience great difficulty in

reading 26

According to the phonological model a deficit in a lower-order linguistic (phonologic) function

prevents access to higher-order processes and to the ability to draw meaning from text The

problem is that the person cannot use his or her higher- order linguistic skills to access the

meaning until the printed word has first been decoded and identified Suppose for example that

a man knows the precise meaning of the spoken word apparition however until he can decode

and identify the printed word on the page he will not be able to use his knowledge of the

meaningof the word and it will appear that he does not know the words meaning 26

1232 Cerebellar Theory

The normal pattern of cerebellar asymmetry is anomalous in dyslexia 13 The cerebellar deficit

theory suggests that the automatisation of cognitive processes and motor control in the

cerebellum are disturbed in individuals with dyslexia14 15 62 Evidence for this theory comes

from poor dyslexic performance in coordination balance and time estimation The cerebellar

theory is related to cerebral dominance (greater proficiency of each cerebral hemisphere in

acquisition and performance of certain specific functions) and lateralization (one-sided location

of brain function)13

Physiologically the brain operates cross-laterally thus the left part of the brain controls the right

side of the body and vice versa The human brain is normally asymmetrical due to the

26

specialized functions in the left and right brain However the dyslexic brain appears to be more

symmetrical than the normal brain The consequence of this is that both sides of the brain may

be competing to handle language and therefore less efficient in handling processing language

The left hemisphere is especially important for the production of language The right hemisphere

also has some language capabilities but has no ability to analyse speech sound Secondly the

cerebellum plays a role in motor control and therefore in speech articulation It is postulated that

retarded or dysfunctional articulation would lead to deficient phonological representations

Furthermore the cerebellum plays a role in the automatization of tasks such as driving typing

and reading A weak capacity to automatize would affect among other things the learning of

grapheme-phoneme correspondences 13-15 62

1233 Visual Deficit Theory

For many years a major view was that reading difficulties are caused by dysfunction in the

magnocellular subsystem 65-67 The visual theory does not exclude the phonological deficit but

emphasizes a visual contribution to reading problems in some dyslexics The biological basis of

the proposed visual theory is derived from the division of the visual system into two sub-systems

which carry information to the visual cortex the transient and the sustained systems62

Anatomically the magnocellular pathway receives input from both rods and cones across the

retina and extends from ganglion cells in the retina to the two ventral layers of the lateral

geniculate nucleus (LGN) The large cell bodies in these two layers give the pathway its name It

projects via the visual cortex and the dorsal or ldquowhererdquo stream to the parietal cortex The

parvocellular (or sustained) system on the other receives input from cones and therefore mainly

the central retina and runs from retinal ganglion cells to the smaller cell bodies of the four dorsal

layers of the LGN It projects via the visual cortex and the ventral or ldquowhatrdquo stream to the

infero-temporal cortex and mediates colour vision and detection of fine spatial details61 The

magnocellular system is the first to respond to a stimulus and relies on further detail from the

parvocellular system (Table 12)

27

Table 12 Features of the Magnocellular and Pavocellular Systems2168

Magnocellular(transient) System Pavocellular (sustained) SystemLarge cells Small cells

Dim light Bright light

High contrast sensitivity Low contrast sensitivity

Low spatial frequency High spatial frequency

Peripheral Central

Right brain Left brain

Midbrain Cortical

Non-cognitive Cognitive

Emotional Logical

Hyperopia Myopia

Gross depth perception Stereopsis

Achromotopsia Colour vision

Fast transmission Low transmission

Sensitive to large stimuli Sensitive to small stimuli

The transient system is vital to the timing of visual stimuli and plays an important part in

detecting the movement of objects (motion perception) and ultimately their signals are used to

control eye and limb movements made in relation to visual targets1521 Impaired function of

magnocellular pathway will lead to destabilization of binocular fixation which results in visual

confusion and letters then appear to move around

Dyslexicsrsquo eyes are unsteady when they are attempting to view small letters hence their vision is

unstable they tend to make visual reading errors15 There is a tendency to superimpose letters on

top of each other and mis-sequence letters in a word1621 Evidence for magnocellular

dysfunction comes from anatomical studies of dyslexic individuals showing abnormalities of the

lateral geniculate nucleus1562 psychophysical and neuroimaging studies 62 64 showing decreased

sensitivities in the magnocellular range (low spatial frequencies and high temporal frequencies)

in dyslexics

28

A deficit in the magnocellular system slows down the visual processing system resulting in an

unstable binocular control inaccurate judgements of visual direction a tendency to superimpose

letters and mis-sequence of letters in a word16 21 Some studies65-67 have shown evidence in

support of the magnocellular deficit theory of dyslexia

1234 Auditory Processing Theory

The auditory processing theory specifies that the deficit lies in the perception of short sounds and

fast transitions (called the ldquorapidrdquo or ldquotemporalrdquo auditory processing deficit) Such a

characterization of the auditory dysfunction is consistent with the magnocellular theory since

magnocellular cells are particularly sensitive to high temporal frequencies60 62

Proponents of the auditory processing theory hypothesized that impaired perception of brief

sounds and transitions would be particularly detrimental to speech perception hence would

undermine the development of the childrsquos phonological representations Contrary to this

hypothesis Rasmus60 reported that recent studies have now adequately confirmed that there is

no reliable relationship between performance on rapid auditory processing tasks and speech

categorization and discrimination neither is there a reliable relationship between any auditory

measure (speech or non-speech) and more general measures of phonological skill or reading

ability even when assessed longitudinally A possibility is that the most auditorily impaired

dyslexics also have severely impaired phonology and reading although the reverse is not

necessarily true 60

Despite the fact that evidence exist in support of the theories and that some of the deficits found

in affected individuals are correlated with reading and spelling they may not be causally

associated with dyslexia However results from genetic research may have the potential to help

delineate which cognitive and neurophysiological processes are causally related14

124 Cultural Differences in Dyslexia

The recognition of dyslexia as a neuro-developmental abnormality has been influenced by the

belief that it is not a specific diagnostic entity because it has variable and culture-specific

29

manifestations 69-71 Recent neuro-imaging studies39-42 have demonstrated that impaired reading

of alphabetic scripts is associated with dysfunction of left temporo-parietal brain regions

However it has been reported by Siok et al in two separate studies69 70 that in contrast to the

assumption that dyslexia in different languages has a universal biological origin their study using

functional magnetic resonance imaging with reading-impaired Chinese children and associated

controls showed that functional disruption of the left middle frontal gyrus is associated with

impaired reading of the Chinese language (a logographic rather than alphabetic writing system)

The neural connections involved in reading and reading disorders may vary in different languages

because of differences in how a writing system links print to spoken language69 70 In Chinese

for example graphic forms (characters) are mapped to syllables which differ markedly from an

alphabetic system (such as English) in which graphic units (letters) are mapped to phonemes

These differences can lead to differences in how reading is supported in the brain69 70 Readers

of Chinese show relatively more engagement of visuo-spatial areas and left middle frontal

regions for verbal working memory presumably for recognizing complex square-shaped

characters whose pronunciations must be memorized instead of being learned by using letter-to-

sound conversion rules69 70

In two separate studies Siok Perfetti and Tan69 and Siok Zhen Charles Perfetti and Tan 70

studied language-related activation of cortical region of Chinese dyslexic school children using

the functional magnetic resonance imaging and found a reduced activation in the same left

middle frontal gyrus region in Chinese dyslexics Chinese dyslexics by contrast did not show

functional or structural differences from normal subjects in the more posterior brain systems that

have been shown to be abnormal in alphabetic-language dyslexics69-70 The authors concluded

that the results of the studies suggested that the structural and functional basis for dyslexia varies

between alphabetic and non-alphabetic languages and that the findings provided an insight into

the fundamental patho-physiology of dyslexia by suggesting that rather than having a universal

origin the biological abnormality of impaired reading is dependent on culture

The study by Paulesu Demonet Fazio and McCrory71 gave another perspective Using the

positron emission tomography to study brain activity of dyslexics from three countries (Italian

French and English) the authors found the same reduced activity in a region of the left

30

hemisphere in dyslexics from all three countries with the maximum peak in the middle temporal

gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus

Their study showed that there was a universal neuro-cognitive basis for dyslexia and that

differences in reading performance among dyslexics of different countries are due to different

orthographies Languages with transparent or shallow orthography (such as Italian) the letters

of the alphabet alone or in combination are in most instances uniquely mapped to each of the

speech sounds occurring in the language Learning to read in such languages is easier than in

languages with deep orthography (such as English and French) where the mapping between

letters speech sounds and whole-word sounds is often highly ambiguous 71

125 TreatmentIntervention

Individualized instruction of dyslexics emphasizing increased phonologic awareness can have a

favorable long-term effect on academic achievement Individualized instruction aimed at

increasing phonologic awareness decoding skills sight word vocabulary and reading

comprehension are particularly helpful to school aged children experiencing difficulties with

reading Most importantly intervention (provided by expert teachers) should be initiated early50

Reading disabilities require lifelong assistance and optimal management strategies differ

depending on the patientrsquos age and circumstances In early childhood the focus is on

remediation of reading often with an emphasis on increasing phonologic awareness Other

strategies include using audio books and modified homework assignments For secondary and

college students intervention focuses on accommodations which include extra time for reading

tape recorders in the classroom audio books instruction in word processing and the use of a

spell-checker (poor phonemic association also causes problems in spelling)3450

126 Role of Optometry in the Multidisciplinary Management of

Dyslexia

The role of the optometrist in the multidisciplinary management of dyslexia starts with an attempt

31

to identify characteristics which may indicate that the child may be dyslexic A complete visual

examination should be conducted Any vision anomalies detected should be corrected and a

prompt referral should be made to the appropriate specialist in the diagnosis of dyslexia and a

follow up should be made by the optometrist

A comprehensive visual evaluation should include1 72

1 Evaluation for a sensory problem or refractive anomaly

2 Evaluation for binocular vision problems

3 Evaluation for ocular pathology

4 An evaluation of visual motor skills (required for drawing and handwriting) eye movement

control skills and visual imagery skills

5 Evaluation of visual information processing including visual spatial skills

(rightleft discrimination) visual analysis skills (matching and discrimination skills)

The role of the optometrist in the management of reading dysfunctions is to identify existing

vision disorders which can be achieved by evaluating the following aspect of vision functions

1261 Case History

A detailed case history is paramount for children who present with reading dysfunction A

thorough investigation of the pre-natal peri-natal and post-natal risk factors is important to

establish the possibility of major developmental delay For example maternal history of

alcoholism may indicate foetal alcohol syndrome Prematurity (early birth date of less than 37

weeks or low birth weight of less than 2500g) is also a factor in later learning A working

knowledge that may not necessarily mean expertise in all related areas may be adequate for the

optometrist72

1262 Refractive and Binocular Functions

A thorough investigation and compensation for refractive and binocular vision functions should

be undertaken4472

1263 Visual-Motor Skills

32

VisualndashMotor integration is the ability to coordinate visual information-processing skills with

motor skills44 It is a very vital skill in academic achievement4472 Children suffering from visual-

motor dysfunction have difficulty copying written work accurately and efficiently Difficulty

with copying can reduce the speed and accuracy of writing which subsequently affects

learning4472 The Beery Test for Visual-motor Integration is particularly useful for the primary

care optometrist It requires only a few minutes to administer with an added advantage that the

scoring guide provides many examples of the pass-fail criterion for each item tested72

1264 Visual-Spatial Analysis

ldquoThe visual-spatial skills allow the child to make judgments about location of objects in visual

space in reference to other objects and to the individuals own body It develops from awareness

within the individualrsquos body of concepts such as left and right up and down and front and back It is important for good motor coordination balance and directional sensesrdquo 72 The Rosnerrsquos

Visual Analysis Skills programme is useful for the assessment of both visual-motor skill and

spatial analysis72

1265 Short- Term Visual Memory (Visual-Motor recall)

The visual memory test assesses the ability to recognize and recall visually presented material 4473 The ability to copy written material accurately is more or less a matching process when the

written materials are close (such as copying from workbook to paper) When the reading

material is separated from the actual visual-motor task such as copying from chalkboard to

paper the process becomes more complex An example of short-term visual memory skill

assessment test is the Getman-Henderson-Marcus Test of Visual recall72

1266 Auditory Analysis Skills

Screening for auditory-perceptual skill at the primary care level is essential as the ability to

analyze spoken language into separate sounds and sound sequence is directly related to reading

and spelling success Examples of tests to measure auditory perceptual skills are the Test of

33

Auditory Analysis Skills (TAAS) (which is easy to administer and score) and a more complex

one Test for Auditory-Perceptual Skills72

127 Optometric Treatment Options

The optometric treatment options for the correction of vision defects include the following

options

1271 Correction of Vision Defects

1 Compensation for refractive error

2 Treatment of visual efficiency deficits with lenses prisms and vision therapy

3 Treament of vision information processing deficits with vision therapy commencing with

visual spatial orientation then visual analysis and concluding with visual-motor

integration

4 Referral to another health care professional educational system and or psychologist

should be considered at any time if underlying physical or neurological problems cognitive

deficits or emotional disorders are suspected1 50 72

The expected outcome of an optometric intervention is an improvement in visual function with

the reduction of vision anomalies associated with reading which makes educational

intervention easy1 Solan47 proposed that the role of optometrists should not end with

correcting vision anomalies in dyslexic patients but that optometrists should participate in

community services to assist with the developmental environment of those whose genetic

make-up may predispose them to having reading difficulties

1272 The Use of Tinted Lenses and Coloured Overlays

The patients rely on professionals to attend to their vision care needs Consequently it is

imperative that we practice at the highest level of professionalism and ensure that anything we

prescribe for our patients are supported with credible research

As reported by Evans74 75 Olive Meares in 1980 suggested that some childrenrsquos perception of

text and certain cases of reading difficulties were influenced by print characteristics which

34

subsequently resulted in visual perceptual anomalies such as words blurring doubling and

jumping Subsequently Meares and Irlen proposed that ldquoScotopic sensitivity syndrome or

(Meares -Irlen syndrome) ndash a syndrome of visual symptoms and distortion can be alleviated

with coloured filters Meares and Irlen claimed that the coloured overlays improve reading

ability and visual perception increase sustained reading time and eliminate symptoms

associated with reading such as light sensitivity eyestrain headaches blurring of print loss of

place and watery eyes75 This type of symptoms is reported both by people who experience

frequent severe headaches of the migraine type as well as some people with dyslexia76

Following this initiative Irlen developed a proprietary system in countries like the United States

of America United Kingdom and Australia to detect and treat Meares-Irlen syndrome Meares-

Irlen syndrome is reported to be treated with coloured filters either coloured sheets (overlays)

placed on the page or colored lenses Computer users may attain a similar effect by changing the

colour of the screen background and or text while people that consistently work under the same

lighting conditions can benefit from the colour therapy by simply varying the colour of the

illuminating light75

The earlier studies of Irlen was criticized for its lack of published double masked studies with a

placebo control to support her claims that the colour that helped had to be specific for each

person76

According to Lightstone76 the Intuitive Colorimeter (developed by Professor Wilkins) of the

Medical Research Council in the United Kingdom was the first to be used to conduct a masked

trial With this instrument it was possible to determine the optimum colour for the spectacle lens

without placing lenses in front of the eyes and could be done in a way that the subject is unaware

of the exact colour they had selected At the end of the study it was felt that the use of colour

was credible and was then included as part of a routine assessment of people with specific

learning difficulties at the Institute of Optometry London76

As described by Lightstone76 the Intuitive colorimeter is basically a large box which the patient

or subject looks into The space that the subject looks at is entirely diffused by a coloured light

and so creates the effect of wearing coloured lenses There are no other colours in the field of

35

view and so no reference colours are available to help the subject decide the exact hue (colour) of

the light that they are viewing This encourages rapid colour adaptation The person operating

the colorimeter changes all aspects of the colour that the subject is viewing and the hue

saturation (depth of colour) and the brightness of the colour can be varied independently of each

other Although the procedure is subjective the technique shows whether there is a specific

colour that gives relief from the symptoms If the effect is placebo the results tend to be variable

and no specific colour can be selected as the optimum colour75 76

An advantage of the Intuitive colorimeter is that because of colour adaptation the person is

unaware of the exact color of the light shining on the text74 However a major demerit with

double-masked trial was that the results were purely subjective and there were no objective

measurement of any improvement with the colour 76

128 Potential Mechanism for Benefit from Tinted Lenses

Some theories have been proposed to be the mechanism of benefit from tinted lenses However

Evans 49 suggested that these hypotheses have not been able to account for the high degree of

specificity of the required colour which has been emphasized by Irlen and substantiated by

double-masked randomized placebo-controlled trials

The theories are

a Pattern Glare The basis of the theory on how pattern glare explains the benefits of tinted

lenses is that certain cells within the visual cortex are hypersensitive and so causes

discomfort while viewing patterns of repeating black and white lines on a page 75 Since

some of the neurons in the visual cortex are sensitive to specific colours varying the colour

of the illuminating light may change the pattern of excitation within the cortex This could

account for the benefit from specific coloured filters From this it has been hypothesized

that by changing the colour of the input the stimulus is transmitted to other cells that are

not hypersensitive and so prevent the distortions and discomfort occurring 7576

b Syntonics Syntonics is a colour vision therapy system used by some optometrist for the

treatment of several conditions including strabismus amblyopia and reading disorders

36

The therapy involves subjects viewing a light source covered by a colored filter The theory

is that the colour influences the endocrine system through the inferior accessory optic tract

The colour seems to be chosen on the basis of a hypothetical connection between an alleged

autonomic nervous system imbalance and the type of heterophoria Treatment is said to

result in improved visual fields7475

c Placebo Effect The placebo effect is one of the possible mechanisms for the benefit from

tinted lenses People may believe that a treatment works simply because it is claimed to be

effective74

Williams and Kitchner77 conducted a literature review on the use of tinted lenses and colored

overlays for the treatment of dyslexia and other related reading and learning disorders and

concluded that

1 there was evidence that the symptoms associated with Irlen syndrome are related to

identifiable vision problems such as binocular and oculomotor problems and that condition

returned to normal when treated with lenses prisms or vision therapy When patients

exhibiting the Irlen syndrome were treated with vision therapy the symptoms were relieved

and therefore did not demonstrate the need for colored lenses

2 most investigators did not control for the presence of vision anomalies

3 the results of prospective controlled research on the effectiveness of tinted lenses or colored

overlays varied

4 the results of testing used to determine the most appropriate color were not repeatable

5 the effects of spectral filters and colored overlays were not solely a placebo Colored

overlays and tinted lenses are not cures for dyslexia but may be useful reading aids for

some individuals with reading difficulty

6 there was a lack of agreement about the best ways to evaluate patients for the presence of

Irlen syndrome

The American Optometric Association78 issued a policy statement on the use of tinted lenses and

colored overlays which states as follows

37

1 Undetected vision problems may be a factor in individuals who exhibit the symptoms of

the Irlen Syndrome A comprehensive eyevision examination with particular emphasis

on accommodation binocular vision and ocular motor function is recommended for all

individuals experiencing reading or learning difficulties as well as those showing signs

and symptoms of visual efficiency problems

2 The American Optometric Association encourages further research to investigate the

effect that specifically tinted lenses and colored overlays have on visual function related

to reading performance

3 Vision problems are a frequent factor in reading difficulties Ignoring the role of vision

or inadequately evaluating the vision of individuals with reading problems is a disservice

which may prevent the person from receiving appropriate care

In concluding Lighthouse76 advices that until more is known about prescribing specific colored

lenses optometrists should be certain to offer the patient the best advice According to Wilkins 79

some 20 of unselected children in mainstream education (not simply those with reading

difficulties) find colored overlays of benefit and the benefit is pronounced in 5 Wilkins79

emphasized that ldquocolored filters do not provide a treatment for dyslexia ndash they provide a means of

avoiding the visual stress with which reading is sometimes associatedrdquo

129 SUMMARY

Dyslexia is a neuro-developomental condition that impacts on a persons spelling

comprehension writing and reading and not only limits their academic career but also impacts

on all areas of life that require these skills Its diagnosis management and treatment require a

multidisciplinary team as it can present with many characteristics which requires careful

classification While it is not always possible to identify the cause it is generally considered to

be a neurological condition with a genetic origin It affects up to one fifth of Caucasians with

limited studies being done on other ethnic groups including Africans

The aim of the study was therefore to investigate the prevalence of vision defects in an African

38

South African population of dyslexic schoolchildren with an attempt to identify high risk

visual conditions that may be prevalent It was hypothesized that there was no relationship

between vision defects and dyslexia

The reading process requires learning to transform printed letters on a page to words that have

meaning Words are made up of phenomes which are represented by letters and it is the

inability to break words into parts that result in the manifestation of dyslexia Its characteristics

include letter reversal faulty pronunciation difficulty with word retrieval and learning to talk

As all visual stimuli is transmitted through the eye there is an association between ocular

conditions and problems associated with letter words and reading hence the involvement of

optometrists The primary role of optometry is to identify and compensate for visual defects

that may constitute an impediment to reading Evaluation by an optometrist is therefore

important to assess and provide advice or devices where appropriate

Initial identification usually takes place by teachers or parents and may result in the child

being sent to a school equipped to deal with learning disabilities Psychologists diagnose

dyslexia using some range of psychometric tests The advent of imaging techniques such as

positron emission tomography and functional magnetic resonance has aided research in

dyslexia as these techniques enable the study of dyslexic brain activity as a reading task is

performed The two broad categories of dyslexia one being due to a genetic neurological

defect and the other being due to a secondary incident that impacts on the brain There are a

number of theories regarding the underlying causes the only theory related (still controversial)

to visual system being the visual processing theory

Chapter One provided the study aims objectives hypotheses to be tested background

information on dyslexia and rationale for the study while Chapter two will review the related

literature on dyslexia and vision

39

CHAPTER TWO LITERATURE REVIEW

21 Introduction

The relationship between vision and dyslexia has been a subject of controversy in optometric

and ophthalmologic literature A review of existing literature 1-32 on the area of vision and

dyslexia reveals that the studies conducted to investigate visual functions in the dyslexic

subjects do not establish a causal relationship between vision and dyslexia but tend to

establish associations between dyslexia and certain vision variables The literature review also

shows that there exists a dichotomy between the ophthalmologists and optometrists on the

subject of dyslexia and vision The difference in perspective can be seen from the style of

report in studies conducted by optometrists 5683-849198100-101 and by ophthalmologists 2-4 85 95

99102 Optometrists maintain that although vision may not be considered as a cause of dyslexia

compensating for visual defects make reading easier for the dyslexic child Ophthalmologists

hold the contrary view that vision does not in any way contribute to dyslexia Dyslexia has

also been thought to be a language-based learning disability80 Consequently from the speech-

language pathologistsrsquo perspective ldquoa child with a speech deficit may also have an emotionally

distorted perspective and a child whose classroom behaviour is hard to control may also be

unable to readrdquo The role of the psychologists is to perform psychometric assessments of the

dyslexic child32 The educator is primarily involved with special educational intervention for

the dyslexic child

Flax 22 proposed that the role of visual factors in dyslexia ldquodepends on the definition of vision

that is consideredPrime meaning that if vision is narrowly defined as visual acuity or clarity of

sight it is unlikely that there would be any relationship between vision and dyslexia If other

peripheral visual functions such as fusion convergence refractive errors and accommodation

are considered there is likely to be a relationship but then questioned the possibility of vision

being a causative factor Flax22 reiterated that the most important aspect of vision that is

closely linked to dyslexia is when vision is defined to include perceptual and integrative

aspects of vision It was concluded that ldquovision difficulties are part of the dyslexic

40

syndrome22

From a similar perspective Solan81 suggested that vision anomalies are contributory and

frequently hinder responses to educational intervention He emphasized that in cases where

vision anomalies are factors involved in the reading disability the effort required to maintain

clear single binocular vision decreases the efficient organization of the visual cognitive

response

This review has been structured in a way that an overview of a study will be made when it is

first cited Subsequently such studies will be cited only with references to the vision variable

being reviewed Only the studies that relate directly to the topic of the present research such as

visual acuity refractive errors near point of convergence ocular alignments accommodation

functions and vergence reserves are reviewed

It is also important to note that there are many variations in the results reported by different

authors In view of this the findings are thus presented on an individual basis Various studies

have attempted to study the relationship between dyslexia and vision and have examined the

different aspects of vision suspected of affecting reading ability However the major

drawbacks that plague the researches in the subject are Firstly intelligence quotient is a factor

to consider when defining dyslexia However only a few studies587-89 consulted have

considered intelligent quotient in their subjectsrsquo selection

Another area worth mentioning is the fact that some of the studies lacked control group which

made the reports difficult to assess and utilize In addition some researchers failed to classify

some visual functions such as binocular coordination and refractive errors thereby making it

difficult to evaluate the impact of a particular vision variable on reading ability Furthermore

some studies failed to indicate the eye examination techniques used in their protocol which

makes the replication of such studies difficultGiven the inherent limitations with studies

conducted to investigate vision in dyslexic children these studies are important in guiding the

teacher and clinicians on the appropriate intervention for the child

22 Visual Acuity

41

Vision plays an important role in the reading process as the ability to see printed material is

crucial in reading Reading efficiency and speed starts to decrease when the print size is less

than three times larger than an individualrsquos visual acuity threshold A marked bilateral

reduction in visual acuity can definitely limit reading performance but it is unknown if

deficient visual acuity characterizes students who have difficulty to read82

Available studies3-58384 that have attempted to see how visual acuity affects reading

performance are presented starting with the first three studies in this review that found a higher

prevalence of reduced visual acuity among dyslexics compared to normal readers

In 1994 Latvala Korhoenen and Penttinen 3 assessed the ophthalmic status of fifty five (55)

dyslexic and fifty (50) control subjects in an age gender and social class matched study in

Finland The subjectsrsquo school grade and intelligence quotient were not indicated Since

intelligence quotient is a major factor in the definition of dyslexia the lack of information on

intelligence quotient may lead to the questioning as to if the population studied were truly

dyslexics The authors assessed visual acuity using the Snellen chart and found that for the

control group all subjects had visual acuity of 07 (69) while in the dyslexic group two

children (36) had bilateral visual acuity of less than or equal to 07

A study to examine the lsquoophthalmologicalrsquo status of school children with dyslexia in Norway

was conducted by Aasved 4 in 1987 Visual examinations were performed on two hundred and

fifty nine dyslexic children The number of the participants from the control group was not

clearly stated The investigation techniques used was also not detailed As a result it may be

impossible for other researchers to replicate the study However Aasved 4 reported that the

dyslexic population had a higher prevalence of reduced visual acuity than the normal readers

did It was concluded that although there was no causal relationship between eye

characteristics and reading difficulties eye abnormalities should be corrected when detected in

dyslexic children

42

Evans et al 5 in 1993 studied some sensory and refractive visual factors in 39 dyslexic and 43

control subjects aged between seven years six months and twelve years three months in the

United Kingdom Participants from both groups had intelligence quotient of above 85 The

authors found that the dyslexic group had a significantly worse near and distance binocular

visual acuity the inter-ocular difference was not statistically different in the two groups and

the difference between the best monocular and the binocular visual acuity was not statistically

different between the two groups at distance or near They claimed that the ldquoreduced visual

acuity may not have been due to refractive defectsPrime Apart from refractive errors a reduced

visual acuity may arise from pathological conditions such as cataract and glaucoma

The following two studies reported similar prevalence of visual acuity worse than 69 In 1980

Hoffman83 investigated the incidence of vision difficulties in learning disabled children in the

United States of America Vision evaluations were performed on one hundred and seven

children with learning problems who were referred to the clinic for vision care from educators

and psychologists The subjectsrsquo mean age was eight years eight months As a comparison

(control) group twenty five patients between the ages of five and fourteen were selected from

the general optometry clinic The prevalence of visual acuity poorer than 69 was reported to

be 1028 among the learning disabled subjects The subjects from the control group were not

appropriately selected not representative enough and were not age-matched although the issue

of the comparison group being inappropriately selected was acknowledged by the author

Another limitation with the study was that no information was given on the eye examination

procedures followed This may be problematic as it makes comparison with other studies and

replication of the study difficult

Sherman84 conducted a study to ldquorelate vision disorders to learning disabilitiesrdquo in the United

States of America in 1973 Thirty-nine boys and eleven girls participated in the study The

children were diagnosed as having learning difficulties by the referring psychologists Their

43

ages ranged from six to thirteen years The technique used to measure visual acuity was not

indicated but it was reported that five children (10) had visual acuity poorer than 69

Other studies reported the prevalence of visual acuity to be similar between the control and

reading disabled group and include the following studies In 1985 Helveston Weber and

Miller85 (from ophthalmology) studied visual functions and academic performance in first

second and third grade children in the United States of America One thousand nine hundred

and ten children participated in this study The Snellen chart and the Jaegar print were used to

assess distance and near visual acuity respectively The authors reported that 94 of the

children tested had normal visual acuity and that visual acuity was not related to academic

performance in a positive way Although the study may be credited with a large sample size a

possible limitation with the application of the findings of this study was that the passfail

criteria were arbitrarily chosen by the examiners ldquoon the basis of previous clinical experiencerdquo

without references to other research studies Secondly the authors failed to present the results

of the statistical analyses of the relationships between vision functions and reading

Grisham and Simon82 conducted a review of literature on refractive error and reading process

and reported that there are abundant and reliable scientific evidence that shows a higher

prevalence of vision defects among reading disabled children than among normal readers It

was emphasized that there are cases where prescription of glasses has improved academic

performance Their analysis revealed no statistically significant difference in distance visual

acuity among normal and disabled readers but that near visual acuity may be an index which

distinguishes some children who are having difficulty to read and those who do not

It is important to note that since reading is a near point task a deficient near visual acuity may

be more contributory to poor reading than reduced distance acuity Consequently Grisham

and Simons82 stated that ldquodeficient distance visual acuity would not be expected to affect the

44

reading skills of children unless substantial reading instructions take place on the chalk board

It is possible that distance acuity is a factor in the early grades when reading is first

introducedrdquo

In 2007 Grisham Powers and Riles86 studied visual skills of 461 poor readers in California

high schools The participants mean age was 154 years and there was no control group in the

study Participants for the study had been identified by the schools as poor readers The vision

functions measured were visual acuity near point of convergence ldquoconvergence and

divergence and fusional rangesrdquo accommodative amplitude and accommodative facility No

information was given on IQ but it was mentioned that the basis for referral of students for

assessment was poor reading performance which was determined by the school and defined as

reading two grade levels or more below grade level Visual acuity was assessed using the

Snellenrsquos chart at six meters The authors reported that 568 of the participants had visual

acuity of 66 in both eyes 261 had visual acuity of 69 in each eye while 172 had visual

acuity of worse than 69 in either eye

Another comprehensive study conducted in 2007 was reported by Kapoula Bucci and

Jurion87 Convergence and divergence functions in 57 dyslexic and 46 non-dyslexic age-

matched children were studied in French dyslexic children The mean age of the dyslexic

subjects was 113 years the mean IQ was 105 and the mean reading age was 891 The mean

age for the control group was 107 years The IQ score of the control subjects was not

indicated The vision functions performed included visual acuity stereo acuity and cover test

near point of convergence and ldquomeasurement of convergence fusional amplitude (divergent or

convergent) Unfortunately the authors did not detail the testing procedures for the vision

functions investigated but reported the results of the study It was reported that all children had

normal visual acuity

In 2008 Alvarez and Puell 88 studied accommodation functions in school children with reading

difficulties in Madrid Spain Eighty seven poor readers (mean age 92 years) and 32 control

(mean age not indicated) participated in the study all in grades three through grade six

Relative accommodation amplitude of accommodation and accommodation facility was

assessed

45

The most current literature reviewed was reported by Bucci Gignac and Kapoula 89 in 2008

The authors studied poor binocular coordination of saccades in dyslexic children in France Of

particular interest to the current review is the assessment of visual acuity and refraction

(though was assessed under cycloplegia) heterophoria near point of convergence and fusional

vergence reserves The study participants comprised of 18 dyslexic children (mean age 114

years and mean IQ was 105) The control group comprised aged-matched non-dyslexic

children mean age 11 2 years There were no details on the method of assessment of visual

acuity and refraction but it was indicated that a cycloplegic refraction was conducted

However the authors reported that all children had normal visual acuity but the results for

refractive errors were not reported

In 1991 a study on stereopsis accommodative and vergence facility was conducted by

Buzzelli 6 in the United States of America Thirteen normal readers of average age thirteen

years three months and thirteen dyslexics average age of thirteen years four months

participated in the study Both groups were matched for gender age and intelligence quotient

The study design was comprehensive The Snellen chart was used to assess the visual acuity

Buzzelli6 found that the dyslexics did not perform worse than the control subjects in visual

acuity

Ygge Lennerstarnd Axelsson and Rydberg2 studied visual functions in a Swedish population

of dyslexic and normally reading children in 1993 Eighty six 9-year- old children matched to

controls with regard to age gender class in school and intelligence participated in the study

VA was assessed with ldquoordinary optotypes chartsrdquo They reported that the subjects from the

control group had a better visual acuity than the dyslexic group at both distance and near and

that the results showed statistically significant differences (distance at p =003) and (near at

p=0005)

A comparative study of dyslexic and normal readers using orthoptic assessment procedures

was conducted by Goulandris McIntyre Snowling Bethel and Lee90 The study was

conducted in 1998 in the United Kingdom and comprised 20 dyslexic and 20 chronological and

46

reading age-matched control participants A unique aspect of the study was that the distance

VA was assessed using the Cambridge Crowding Cards This test according to the authors

was devised to evaluate letter recognition acuity in conditions of lsquocrowdingrsquo when other letters

surround the target letter and that the test is designed to ensure that abnormalities of vision

which would not be revealed reliably using single targets were detected The near acuity was

assessed using the Snellenrsquos chart The use of different VA chart at distance and near may

raise a concern of standardization They reported that the prevalence of distance and near VA

(using a 69 criteria) was similar between the groups

In 1992 Evans Drasdo and Richards91 investigated optometric correlates of reading disability

in 10 children aged between eight and fifteen years who were referred to the optometry clinic

by the child psychologist The authors reported only the statistical values mean binocular

distance visual acuity of + 091 and mean binocular near visual acuity of + 089

47

23 Refractive Errors

In the study by Alvarez and Puell 88 no information was given on the protocol used to assess

refractive error but the refractive error results were reported For the poor readers mean

spherical equivalent refractive errors for the right and left eye were 020 06 and 020 06

respectively Fifty-seven (655) children were emmetropic In the control group mean

spherical equivalent refractive errors for the right and left eye were - 020 08 and - 014

080 respectively Twenty (645) children were emmetropic In contrast Evans et al 91

reported the mean spherical equivalent refractive error (by retinoscopy) of + 077DS for the

(right and left eye was similar)

231 Myopia

Myopia is characterized by a decrease in distance visual acuity Myopia would usually not be

expected to affect reading skills since students can make the necessary adjustments to their

reading positions However from a clinical standpoint unusually high myopia myopic

anisometropia and astigmatism could influence patients near performance and cause some

discomfort to the child There is no evidence in the literature that myopia is associated with

poor reading In the study by Alvarez and Puell88 57 of the children from the dyslexic

group were myopic compared to 194 from the control group

In 1987 Rosner and Rosner 93 conducted a retrospective comparison of visual characteristics of

data for two hundred and sixty one children with learning difficulties in the United States of

Ameria who had been identified by the schools as manifesting learning difficulties (LD) The

control group consisted of four hundred and ninety six children without learning difficulties

The subjects from both groups were of age range between six and twelve years The children

had been examined at the University of Houston Optometry Clinic The data collection

techniques used was not given but it may be assumed to be routine optometric eye examination

48

protocols because the study was conducted at the university optometry clinic The prevalence

of myopia (minus 050 dioptres) was reported to be 54 in the non-learning disabled patients

while 19 of the learning disabled patients were myopic

One limitation with this study however was that all subjects were patients of an eye clinic

where the two main reasons for referral are reduced visual acuity and school learning

problems Consequently the prevalence of visual defects in subjects selected from such a

clinical setting may have been higher than those from a non-clinical setting

In a review study conducted by Grisham and Simon82 the prevalence of myopia was lower in

the reading disabled group than among the normal readers Similarly Grosvenor 94 analysed

data from several studies and concluded that myopia is consistently associated with good

reading performance One limitation with this analysis however that is the conclusion drawn

from the review was based only on studies that found positive relationships between vision

anomalies and reading ability Grosvenor 94 however remarked that ldquomany authors failed to

find the relationship because they used ineffective methods

One of the earliest studies on the subject of vision and reading difficulty was conducted by

Eames 95 in 1948 in the United States of America He studied refractive and binocular vision

anomalies in one thousand (1000)rdquoreading failures one hundred and fifty (150) unselected

children and five hundred ophthalmic patientsrdquo Comparing the poor readers and the

unselected groups (control) he found the incidence of myopia to be same in both groups (4)

Myopia was defined as -100 diopter sphere There was no statistical analysis in that study and

the data collection techniques were vague although it was indicated that no cycloplegia was

used While the study may be criticized as not being current it is the major and early study to

give an insight into the subject of vision and reading disability

Eames95 and Shearer96 reported a similar prevalence of myopia (4) Shearer96 investigated

ldquoeye findings in children with reading difficultiesrdquo Two hundred and twenty two children

participated in the study The subjects were screened using the Keystone telebinoculars The

prevalence of myopia defined as - 050 dioptres sphere was 4 He concluded that vision

49

functions are not related to reading ability

232 Hyperopia

Hyperopia is often associated with poor reading performance probably through the mechanism

of deficient accommodation and poor motor fusion skills 82 A child with a relatively high

degree of farsightedness may not report blur vision at distance due to compensation for

hyperopia through the accommodation mechanism which also is used to create clear vision at

near This results in overuse of the accommodation system both optically and physiologically

and a subsequent fatigue or spasm of the accommodation system 97

One of the studies that classified refractive error in the literature was conducted by Rosner and

Rosner93 They reported that hyperopia appeared to be more prevalent (54) in learning

disabled than in the non-learning disabled children with a prevalence of 16

In the study conducted by Alvarez and Puell88 287 of the dyslexic group was hyperopic

compared to 161 from the control group Grisham and Simon82 in a literature analysis of

refractive errors and reading process reported that the prevalence of hyperopia among poor

readers is higher than among good readers and that the correction of hyperopia resulted in

improved performance

Grosvenor 94 reported that hyperopia is usually associated with poor reading Similarly in

Eamesrsquo study95 43 of poor readers had hyperopia while for the control group only 12 were

hyperopic The author gave no information on statistical analysis Shearer96 reported the

prevalence of hyperopia to be 16

233 Astigmatism

Clinically uncorrected moderate to high astigmatism can cause discomfort and lead to poor

reading performance at near distance Relatively few studies have attempted to study the

relationship between astigmatism and reading disability The prevalence of astigmatism

reported was higher in the dyslexic group compared to the control group in all the studies

reviewed 39395 These studies include the study by Latvala et al 3 who reported that 36 of

50

the dyslexic subjects were astigmatic (of greater than -100 diopter cylinder) and none from the

control group had astigmatism of that magnitude Eames95 reported the prevalence of

hyperopic astigmatism of more than -100 diopter cylinder among the poor readers to be 6

and 4 in the unselected group while the prevalence of myopic astigmatism was the same

(1) in both groups Rosner and Rosner 93 found a higher prevalence (30) of astigmatism

amongst the non-learning disabled subjects compared to the learning disabled group (27) In

Shearerrsquos 96 study of eye findings in children with reading difficulty the total prevalence of

astigmatism classified as myopic astigmatism (3) and hyperopic astigmatism (3) reported

was 6 There was no control group in this study In the study by Alvarez and Puell 88

astigmatism was detected in seven (8) and eight (92) right and left eyes respectively while

astigmatism was detected in five children (161) from the control group Ygge et al 2

reported that the prevalence of astigmatism was higher in the dyslexic group (28 left eye 25

) compared to the control groups (right eye 183 left eye 243) although there was no

statistically significant difference between the groups (p=025)

234 Amblyopia

Only two studies on the available literature consulted reported on the prevalence of amblyopia

both indicating a higher prevalence amongst the dyslexic group compared to the control group

In the study by Latvala et al 3 the prevalence of amblyopia in the dyslexic group was 36 and

none in the control group had amblyopia Although a detailed prevalence of refractive error

was not presented in that study it was reported that nearly all subjects had visual acuity of 07

In relating refractive error to amblyopia the prevalence of astigmatism in the control group

was 36 and none from the dyslexic group had astigmatism Consequently a low prevalence

of amblyopia may be related to fairly good visual acuities in both groups On the contrary

Rosner and Rosner 93 reported that 4 of the non-learning disabled patients had amblyopia

compared to the 3 found in the learning disabled patients Hyperopia is the refractive error

that is more often associated with amblyopia However this study presented a higher

prevalence of amblyopia (4) but a lower prevalence of hyperopia (16) in the non-learning

disabled patients However a difference of only 1 is not high enough to account for the

difference

51

235 Anisometropia

Anisometropia has been thought to be associated with poor reading skills probably through the

mechanism of poor sensory and motor fusion rather than reduced visual acuity It degrades

binocular coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 82

Three studies2395 reported findings on anisometropia Latvala et al 3 reported a higher

prevalence of anisometropia (of greater than or equal to one diopter) of 6 for the control

group than in the dyslexic group with 36 Similarly Ygge et al 2 found a higher prevalence

of anisometropia in the control group (158) than in the dyslexic group (94) Eames 95

reported that 13 of poor readers were anisometropic while 6 from the control group had

anisometropia According to Grisham and Simons 82 only few studies have considered

anisometropia to affect reading performance The authors concluded that there was insufficient

number of studies to draw firm conclusions from their review

Other studies2598 failed to classify refractive error according to the type These include

studies by Evans et al 5 who investigated some sensory and refractive factors in dyslexia The

distribution of refractive errors was found to be similar in both the dyslexic and control groups

The details of the participants profile was given earlier (section 21) The refractive errors

were assessed by distance retinoscopy and subjective refraction A cross cylinder was used to

measure astigmatism The spherical equivalent refraction was calculated by adding one-half

of the cylindrical components to the spherical component It was not indicated if refraction

was performed under cycloplegia Ygge et al 2 assessed refractive errors objectively using

streak retinoscopy under cycloplegia They reported that the distribution of refractive errors

were similar between the dyslexic and control groups

Sucher and Stewart 98 examined 136 fifth and sixth grade children comprising seventy two

with learning disability and 64 from the mainstream school as the control group The learning

52

disabled children were selected from a classroom of children attending special supervision and

instruction Children from both groups were matched by grade level No information on

intelligence quotient was given The testing protocol used to assess refractive error was not

indicated The authors reported a higher prevalence of refractive error in the dyslexic (208)

than in the control (94) group As indicated earlier refractive errors were not classified

according to the types in the three preceding studies 2598

Hoffman 83 reported the prevalence of refractive error to be 215 There was no report on the

control group and the techniques used to assess refractive errors were not indicated In the

study by Helveston et al 85 only 1 of the participants had refractive error when assessed

objectively with the retinoscope without the use of cycloplegia Refractive errors were not

classified according to the types (myopia hyperopia or astigmatism)

In 1991 Hall and Wick 99 conducted a study to investigate the relationship between ocular

functions and reading achievement in the United States of America One hundred and eleven

children (in grades one through grade six) participated in the study Eleven ocular functions

were investigated including visual acuity saccade eye movements accommodation facility

amplitude of accommodation accommodation posture heterophoria horizontal fixation

disparity vertical fixation disparity near point of convergence titmus and Randot stereopsis

Accommodation facility was measured using the flipper technique The amplitude of

accommodation was determined using the push up to blur technique Heterophoria fixation

disparity and near point of convergence were also measured but the techniques used were not

indicated The age range of the subjects studied was not indicated Using multivariate

correlation they found no statistically significant relationship between the ocular functions and

reading abilities as they noted that the acquisition of reading skills is given by many forces

which include the use of remedial instructions and language skills the role of peer pressure and

innate intelligence

53

24 Near Point of Convergence

Anomalies of convergence is one of the common problems encountered in the optometric

practice especially in the pediatric population and the reports on near point of convergence has

been presented comprehensively in the literatures reviewed

Grisham et al 86 measured the near point of convergence by asking the student if the clown face

target was seen as single when the examiner held it approximately 20 cm in front of the

studentrsquos eyes If so then the student was requested to say when the object appeared to double

as it moved closer to the face The examiner placed one end of an accommodative rule on the

studentrsquos forehead and judged where the two eyes diverged as the target moved toward the

studentrsquos face The measurement was repeated three times The authors found that 846 of

the students had near points at 8 cm or closer which they considered ldquonormalrdquo while 154

had near point of convergence of 9 cm or farther

In the study by Kapoula et al87 the NPC was determined by placing a small pen-light at

3040cm in the mid plane in front of the subject and moving it slowly towards the eyes until

one eye lost fixation The authors reported that the NPC was significantly more remote

(gt10cm) in dyslexics than in non-dyslexics (36 versus 15)

Bucci Gignac and Kapoula 89 followed similar protocol as in the study by conducted Kapoula

et al 87 The near point of convergence was determined by placing a small pen-light at 3040

cm in the mid plane in front of the subject and moving it slowly towards the eyes until one eye

lost fixation It was reported that the NPC was not statistically different between the two

populations the median value was 5 cm for both dyslexic and non-dyslexic children

Latvala et al3 failed to indicate the technique used in assessing the near point of convergence

54

but simply noted that ldquoconvergence ability was measuredrdquo but reported that convergence near

point greater than or equal to 8cm was found in 127 of the dyslexic subjects and 2 of the

control group The authors concluded that the near point of convergence was the only variable

that showed a statistically significant difference between the groups They concluded that

ophthalmic factors should not be overlooked as a contributing factor to dyslexia as they may

constitute part of the dyslexic syndrome and recommended that vision anomalies should be

corrected whenever detected

Shearerrsquos 96 findings are similar to the results reported by Latvala et al 3 He assessed the near

point of convergence using the keystone telebinoculars and reported that 12 of the

participants in his study had convergence problems

In a relatively elaborate study Evans Drasdo and Richards100 investigated accommodative and

binocular functions in dyslexia The subjects were 43 control and 39 dyslexics aged between

seven years six months and twelve years three months An intelligence quotient range of over

85 for the dyslexics and over 90 for the control with the Wechsler Intelligence Scale for

Children (Revised) (WISC-R) was recorded They measured the near point of convergence

using the royal air force (RAF) rule It was concluded that all subjects were able to report a

clear subjective break and recovery and reported no statistically significant difference between

the two groups in either the near point of convergence

Hoffman83 assessed convergence using keystone visual skills tests but grouped the results as

binocular coordination which makes it difficult to analyze and compare while in the study by

Helveston et al 85 98 of the participants had normal near point of convergence

In 2002 Keily Crewther and Crewther 12 investigated the relationship between functional

55

vision and learning to read in Australian school children Two hundred and eighty-four

children (mean age 99 18 years) received a vision screening emphasizing binocular

anomalies associated with discomfort at near (distance and near visual acuity distance vision

ldquochallenged with binocular +1 D lensesrdquo near heterophoria near point of convergence

stereopsis and accommodative facility) Children were classified as normal readers (n = 195)

children with dyslexia (n = 49) or learning disabled children (children who have learning

difficulty with impaired intelligence quotient) (n = 40) It was reported that the near point of

convergence were better in the dyslexic group compared to the normal and the learning

disabled group but found no statistically significant difference between the groups In

constrast Evans et al 91 assessed near point of convergence using the RAF rule and reported a

highly statistically significant relationship between NPC and reading retardation (p=0019)

25 Heterophoria

Heterophoria is a common binocular problem and in clinical practice it is particularly

important when analyzed in conjunction with fusional reserve as it gives an indication of the

zone of comfortable binocular vision

The distance phoria is a clinical measurement of the tendency of the two eyes to deviate from

parallelism when viewing a target usually at six meters When a closer viewing target is

utilized (say at about the individuals reading distance) the resulting near phoria measures the

degree to which the eyes over converges (esophoria) or underconverges (exophoria) to the

plane of the target97 This latent deviation requires fusional vergence to maintain single

binocular vision and at near they involve the accommodation system Smooth functioning of

these systems particularly at near is important for near tasks92

Several studies312858789959698100101 have attempted to investigate the role of heterophorias on

reading ability and all reported remarkably varying findings The assessment of heterophoria

was extensive in the study conducted by Latvala et al 3 Heterophoria at distance was evaluated

56

using the cover test and Maddox rod The prism bars was used to quantify their findings The

Maddox wing was used to assess phoria at near At near they reported a higher prevalence of

esophoria (61) among the control than the dyslexic group (58) a higher prevalence of

exophoria (25) among dyslexics than the control group (122) and a higher prevalence of

vertical phoria (61) among the control than the dyslexic subjects (36) At far distance

they found a higher incidence of esophoria (113) among dyslexics than the control (82)

subjects a higher prevalence of exophoria (2) among the control group than among the

dyslexics (19) and vertical phoria to be more prevalent in the control (41) than in the

dyslexics (38)

Keily et al 12 assessed heterophoria using the Prentice card and reported that there was a slight

but insignificant tendency for the dyslexic subjects to be more exophoric than the normal and

those with learning disability but found no statistically significant difference The test distance

was not indicated

Evans et al 100 in a study of accommodative and binocular functions in 43 control and 39

dyslexics reported that all subjects examined were orthophoric at distance with cover test

except for one dyslexic subject who demonstrated 10 prism diopters right esotropia and two

diopters right hypertropia which changed to 15 prism diopters right esotropia with two prism

dioptre right hypertropia at near At near horizontal phoria results were similar in both groups

Evans Efron and Hodge 101 studied the incidence of lateral phoria in 45 children with specific

learning disability in the United States of America in 1977 The participants were selected from

a psychologistrsquos file The control group comprised of 364 children in second third and fifth

grades attending regular school It was not indicated why there was such a big difference in

the number of participants from both groups The test used in assessing phoria was the

ldquoRocket card which was used in conjunction with red- green spectaclesrdquo They reported a

57

prevalence rate of 1135 in the control group while 222 of the children from the specific

learning difficulty group had heterophoria There was no statistically significant difference

between the groups In concluding it was recommended that phoria conditions may not cause

reading or learning disability but that the presence of such conditions could contribute to visual

perceptual and attention abnormalities

A report by Sucher and Stewart98 contained limited information as the authors did not detail

whether a higher prevalence rate among control (105) than dyslexic (86) was for near or

distance phoria Similarly Eames95 failed to indicate the distance at which the measurement

was made but reported that 33 of poor readers had exophoria and 22 of control were

exophoric but did not indicate whether distance or near

Helveston et al 85 used the Wottring amblyoscope to determine ldquoobjective and subjective angle

of ocular alignmentPrime and reported that 96 of the participants in their study were orthophoric85 The criteria used to classify ocular deviation were not indicated

Another comprehensive report of heterophoria was documented by Shearer96 The subjects

were screened using the Keystone telebinoculars and reported that 1 of the study participants

had vertical phoria (gt1 prism diopter) 2 had esophoria (gt 4 prism diopter) 3 had

exophoria (gt 4 prism diopter) 26 had exophoria at near It was concluded that vision

functions are not related to reading ability Hoffman 83 reported that ninety three (93) subjects

(869) had problems of binocular coordination but failed to indicate whether it was phoria or

tropia

Kapoula et al 87 evaluated ocular alignment using the cover test and reported that at far

distance the median value of phoria was zero for both non-dyslexic and dyslexic children (p =

019) At near the median value of phoria was similar between the two groups (minus2 prism

58

diopters for non-dyslexics and minus15 prism diopters for the dyslexics (p = 019)

Bucci et al89 measured heterophoria at near and far using the cover-uncover test As

documented by the authors exophoria was not statistically different between the two groups (p

= 02) the median was ldquo0rdquo prism diopters for dyslexics and 2 prism diopters for non-dyslexics

No information was given on other aspects of heterophoria

Simons and Grisham92 conducted a literature analysis on binocular vision anomalies and

reading emphasizing studies with positive and negative relationship between binocular vision

and reading The authors concluded that binocular conditions such as esophoria exophoria

restricted ranges of fusion foveal suppression accommodative insufficiency and oculo-motor

disorders at reading range are more prevalent among poor readers than normal readers

In another study Ygge Lennerstarnd Rydberg Wijecoon and Petterson102 investigated

oculomotor functions in a Swedish population of dyslexic and normally reading children in

1993 The study comprised 86 nine -year- old dyslexic children matched to control with regard

to age gender and intelligence Heterophoria was measured using the cover test They

reported that for the dyslexic group ldquo23 pupils exhibited a phoria (264 9 eso- and 14 exo-

phoria)rdquo whereas in the control group ldquo20 pupils had a phoria (232 6 eso- and 14 exo-

phoria)rdquo and that the difference was not statistically significant (p = 034) At near the

incidence of exophoria was 437 for the dyslexic group and 453 in the control group

There was no statistically significant difference (p=055)

26 Strabismus

Several authors34 83-85 90 93102 included the measurement of strabismus in their studies

reporting remarkably varying findings Helveston et al 85 assessed ocular alignment using the

Wottring amblyoscope and reported that only between 2 - 4 of the students had phoria or

59

tropia of more than four prism dioptres Latvala et al 3 reported the prevalence of tropia at near

for the dyslexic group to be 36 and 2 for the control group Hoffman83 assessed ocular

alignment using the keystone visual skills test and reported that 13 subjects (9 exotropes and 4

esotropes) (1214) had strabismus It is difficult to comment if this relatively high

prevalence of strabismus is related to hyperopia as the refractive error prevalence of 215 was

not categorized as to the type In a study by Sherman84 a total of four children (8) were

strabismic classified as three exotropia (alternating) and one esotropia (alternating) Rosner

and Rosner93 in their retrospective analysis reported that in the learning disabled group 51

had esotropia 39 had exotropia while in the non- learning disabled group 8 were

strabismic It was not unexpected to find a higher prevalence of strabismus in this

retrospective study Aasved 4 found that the dyslexics had a higher prevalence of manifest and

latent convergent strabismus than the control subjects

Goulandris et al90 assessed strabismus using the cover test and reported that there was no

statistically significant difference between the two groups in the prevalence of strabismus

Ygge et al 102 reported that at distance manifest strabismus was more frequent among the

dyslexic (8) pupils than the controls (23) but that the difference was not statistically

significant At near the prevalence of manifest strabismus was 8 (all eso-) for the dyslexics

and 35 (all eso-) in the control group

27 Accommodation Functions

271 Amplitude of Accommodation

The accommodation mechanism is extremely important for learning Children who suffer

some anomalies of accommodation are more prone to fatigue quickly and become inattentive

than those who have normal accommodation function According to Flax 22 an ldquoinefficient

accommodation function is a significant contributor to lowered achievement in the upper

grade

60

The findings on various aspects of accommodation functions were diverse Metsing and

Ferreira 103 studied visual deficiencies in children from mainstream and learning disabled

schools in Johannesburg South Africa in 2008 One hundred and twelve (112) children from

two learning disabled schools and eighty (80) children from a mainstream school in Johannes-

burg participated in the study The vision functions examined were visual acuity refractive

status ocular health status accommodative functions (Posture facility and amplitude)

binocularity (cover test vergence facility smooth vergences NPC and ocular motilities (direct

observation) However according to the authors only the results for accommodation

accuracy amplitude of accommodation vergence facility and saccadic accuracy are reported

since only these variables showed statistically significant relationship on the relationship

between the (mainstream and learning disabled) groups Metsing and Ferreira 103 reported

that a high percentage (516 and 533) of low amplitude of accommodation for the right

and left eyes respectively was found in the mainstream group compared to the 291 and

288 in the learning disabled group The relationships between the mainstream group and

reduced amplitudes of accommodation of the right (p=004) and left eyes (p=0001) were

found to be statistically significant (plt005) The relationship between the mainstream group

and reduced amplitude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to

be moderate and that between the mainstream group and the right eyes (Cramerrsquos V= 0286) to

be low

In the study by Grisham et al86 accommodative amplitude was assessed using the

accommodative rule ldquoThe student held an occluder over the left eye The examiner asked if

the student could see the (2030) target at the larger end of the Occlud-A Measure held at about

20 cm from the face If yes then the student was asked whether the target was in focus or ldquonot

blurryrdquo If the student responded that the target was in focus then the target was moved at

approximately 2cm per second toward the studentrsquos face and the student was asked to report

when the target first became blurryrdquo About 638 of the participants had amplitude of

accommodation of 11D which the authors considered equivalent to the expected amplitude of

61

accommodation for the age About 247 had inadequate accommodation amplitude while

115 had ldquoborderlinerdquo amplitude of accommodation

In the study by Evans et al 100 the amplitude of accommodation was measured using the push up

method They reported that the amplitude of accommodation was significantly reduced in the

dyslexic group and that the mean accommodation lag was calculated for each eye but found no

statistically significant difference between the two groups

Alvarez et al 88 assessed monocular accommodative amplitude using the minus lenses method

The subject viewed horizontal line of 2030 letters at 33cm as the examiner introduced minus

lenses in 025D increments until the target first becomes blurred According to the authors the

working distance adjustment used was kept at 250 DS to compensate for minification The

authors reported that monocular accommodative amplitude was significantly lower (p lt 0001)

in the group of poor readers (right eye 91 D 23 left eye 90 D 23) than in the control

group (right eye 105 D 17 left eye 105 D 17)

Helveston et al85 measured the amplitude of accommodation using the push up to blur

technique and reported that ldquoan ability to accommodate more than eleven dioptres was found in

between 90 and 93 of the studentsrdquo Ygge et al 102 assessed amplitude of accommodation

using the RAF rule and reported that the two groups performed similarly A similar technique

was used by Goulandris et al 90 and a lack of statistically significant difference between the

groups was reported

272 Accommodation Facility

In the same study Evans et al 100 reported that the dyslexic group appeared to be slower at a

62

test of accommodative facility The authors discredited the flipper technique as the

unnatural act of altering accommodation without changing vergence They further noted that

the flipper testhellip would still imperfectly reflect the normal visual environment owing to the

absence of a stimulus to vergence and accommodation

Grisham et al86 measured accommodative facility using the 200D lenses flipper technique

and the technique was performed as follows The 2D lenses was flipped each time the

student said the target was ldquoclearrdquo The target for this test was the 2030 target used for

accommodative amplitude The left eye was occluded The student was asked whether the

target was clear when held at about 40 cm If so then the student was told that the test would

be timed and that it should be reported whenever the target was ldquoclearrdquo The examiner then

started a stopwatch and counted the number of times the lenses were flipped in 30 seconds

That number was taken to represent the cycles per minute (cpm) and therefore to represent the

studentrsquos ability to change accommodation over time Based on the criteria used in their study

the authors reported that 76 of the students had adequate monocular accommodative facility

while 236 had ldquoweak accommodative facility of 9 cpm or less and 81 had accommodative

facility of as low as 1 or 0 cpm

In the study conducted by Alvarez et al 88 accommodative facility testing was conducted using

the following procedure The subject wearing his or her habitual correction was seated and

positioned so that the spectacle plane was 40cm from the Bernell Acuity Suppression Slide

(VO9) A 200 D accommodative demand was used with a 2030 test target at 40cm The

patient was asked to view the reduced Snellenrsquos letters 2030 in line 5 and say ldquonowrdquo when the

letters in that line appear clear and single and to report letters missing from lines 4 and 6

during the binocular testing They reported that the binocular accommodative facility values

were significantly lower (p lt 005) in the poor readers group (49 cpm 31) than the control

group (63 cpm 29)

63

Sucher and Stewart 98 assessed binocular accommodative facility at 35 cm using 150 diopter

flippers lens for ldquoone minuterdquo and ldquoused the University of California Berkeley clinic standard

of 20 cycles90s as a ldquocriterion for successrdquo found a higher prevalence of accommodative

infacility in dyslexics (222) than the control group (86)

On the contrary three authors6 12 93 found the control subjects to have better accommodation

facility than the dyslexic subjects The studies are Rosner and Rosner93 found a higher

prevalence of accommodative infacility among the control (10) than the learning disabled

group (9) Similarly Buzzelli6 found that the dyslexic subjects showed better

accommodation facility than the control group To assess accommodation facility ldquopatients

were asked to clear the number 9 on the Bernell Vectogramm Target at 40cm hellip alternately

viewed the target through a + 200DS and a ndash 200 DS lenshellip for 20 cycles) There was no

statistically significant relationship between the two groups

Keily et al12 assessed accommodative facility using +200DS and ndash 200DS flippers lenses

with an N6 fixation target for one minute at 30 cm rather than 40 cm because according to the

authors the children were required to hold the reading material They reported that

accommodative facility was better in normal readers than either of the poor reading groups

In another study Evans and Drasdo104 reviewed ophthalmic factors in dyslexia and commented

that convergence insufficiency accommodative dysfunctions and unstable eye movements are

more common in dyslexic population than in normal readers

Latvala3 simply noted that some aspects of accommodation were difficult to measure and failed

to give any details

64

273 Relative Accommodation

Alvarez and Puell88 measured relative accommodation using a phoropter The subject viewed a

horizontal line of 2030 letters at 40cm with hisher best refractive correction The examiner

introduced minus or plus spherical lenses in 025 D steps binocularly until the patient reported

first sustained blur Negative and positive relative accommodation values were similar in both

groups This was the only study accessed that reported on relative accommodation

274 Accommodation Posture

Evans et al100 assessed accommodation lag using the monocular estimate method of retinosocpy

and reported no statistically significant difference between the dyslexic and the control groups

Metsing and Ferreira103 measured the accuracy of the accommodative response (lag or lead)

objectively using the MEM retinoscopy in normal room illumination and found that the subjects

from the mainstream school demonstrated a high prevalence of lead of accommodation (40

and 405 for the right and left eyes respectively) However in the learning disabled group a

high prevalence of lag of accommodation (98 and 119 for the right and left eyes

respectively) was found The relationships were found to be statistically significant (p lt 005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000)

Sherman84 assessed accommodation amplitude and relative accommodation using minus lenses

and accommodative facility using the flipper techniques The lens powers for the plus and

minus lenses used to assess accommodation functions were not indicated His findings may be

difficult to utilize due to the somewhat unique way of classifying the results as ldquomechanical

problemsrdquo

Flax22 suggested further investigations in the nature of accommodative function in reading

65

disabled children indicating that the use of the push-up method of assessing the amplitude of

accommodation is unreliable due to its lack of sensitivity Flax22 reiterated that

accommodation has a contribution in learning disabilities and that accommodative function

inefficiencies tend to play an increasing role as the youngster moves through school and that

inefficient accommodation function is a significant contributor to lowered achievement in the

upper grade

28 Vergence Facility

Vergences are a clinical measure of an individualrsquos ability to overcome the prismatically

induced optical displacements of a target in space Convergence relates to each eye fixating

closer in space than the real physical location of the target while divergence is the measure of

the eyes to fixate further in space while still maintaining single binocular vision97

Buzzelli 6 assessed vergence facility as follows ldquopatients were asked to align a vertical picture

(box-X-O) slide made of anaglyph material for monocular presentation under binocular

conditions The target measured 21 mm in the vertical meridian and 10mm in the horizontal

meridian The subjects alternately viewed the target through 16 prism base out and 4 prism

base in The test was continued for 20 cyclesrdquo The author reported that the dyslexics

completed the vergence eye movement task significantly slower (300 seconds) than the normal

readers (240 seconds) A statistically significant relationship between the groups was reported

(p lt 005) It was recommended that the possible role of vergence in dyslexia should be

investigated on a larger sample of dyslexic and control groups

Metsing and Ferreira103 assessed vergence facility test at near (40 cm) with the subjects

presented with a vertical row of letters (69) using the 8 PD (BI) and 8 PD (BO) mounted in a

flipper device They reported that the prevalence of poor vergence facility (205) was found

to be higher in the mainstream group compared to the learning disabled group (183) The

relationship between the mainstream group and poor vergence facility (p = 0000) was found

66

to be statistically significant (plt005) A medium to strong relationship was found to exist

between the mainstream group and poor vergence facility (Cramerrsquos V = 0369) It was

concluded that children from mainstream schools are likely to present with poor vergence

facility compared to children from schools of the learning disabled

2 9 Fusional Reserves

The compensatory effect of fusional vergence reserves on phoria is very important as difficulty

with reading is likely to occur when there is uncompensated phoria in conjunction with

reduced fusional vergence reserves 92

A major study was reported by Evans et al 100 which found that negative and positive reserves

were significantly reduced in the dyslexic group relative to the control The ldquoprism vergence

amplitude calculated as the difference between base out and base in break and recovery points

was relatively reduced in the dyslexic grouprdquo They remarked however that several subjects

were unable to appreciate a subjective blur point but the results for the break and recovery

were complete

In the study conducted by Latvala et al 3 the incidence of fusional amplitude greater than or

equal to thirty two prism dioptre at near was higher in the dyslexic (75) than in the control

(61) group At distance they found fusional amplitude greater than or equal to15 prism

dioptre to be higher in the control (122) than in the dyslexic (94) group

Disordered vergence control was studied by Stein Riddell and Fowler 105 in the Uniteed

Kingdom in 1988 The subjects were thirty nine dyslexic children aged eight to eleven years

and a control group of twenty subjects of similar age group The vergence control was

measured using the Synoptophore They reported that two thirds of the dyslexic group had

67

abnormal vergence control According to the authors vergence eye movements never seem to

have been investigated in detail during reading before and that the study was the first in which

eye movement recording have been used to demonstrate differences between dyslexics and

normal readers

According to Grisham et al86 ldquoConvergence and divergence break and recoveryrdquo were

measured at 40 cm with a horizontal prism bar The student was first asked if the clown face

target on the smaller end of a standard optometric measure (Occlud-A-Measure Bernell

Mishawaka Indiana) could be detected If so then the student was requested to say whether

the target appeared single or double Once single vision was obtained the bar was moved

relative to the studentrsquos eye until double images were reported That point was considered

ldquobreakrdquo Then the motion of the bar was reversed until the student reported the image single

and this point was considered ldquorecoveryrdquo This procedure was first performed for base in (BI)

position of the bar over the left eye and repeated with base out (BO) position over the right

eye The authors reported that a large number of students had poor convergence skills 38

break at less than 18 PD and 95 recover at 7 PD or less These values were considered to be

in the ldquoweakrdquo to ldquovery weakrdquo range and remarked that in contrast to convergence lsquomodalrdquo

values for break (14 PD) and recovery (10 PD) were not in the ldquoadequaterdquo range but would be

classified as ldquovery weakrdquo for break and ldquoweakrdquo for recovery As a result very high

percentages of students had poor divergence skills 82 break at less than 20 PD and 60

recover at less than 11 PD

Kapoula et al 87 measured vergence reserves using prism bars at far and near distance for both

groups They reported that the median value for positive fusional vergence was 16 prism

diopters and 14 prism diopters for non-dyslexics and dyslexics respectively at far distance (p

= 012) and 20 prism diopters for both non-dyslexics and dyslexics at near distance For

negative fusional vergence they reported that ldquodivergence amplituderdquo was significantly

different in the two groups for both distances (p lt 0005) At far the median value was 6

68

prism diopters and 4prism diopters in non-dyslexics and dyslexics respectively and 12 prism

diopters and 10 prism diopters respectively at near

Bucci et al 89 measured fusional reserves at far and near using prism bars (base in and base

out) According to the authors orthoptic evaluation of vergence fusion capability showed a

significantly limited divergence capability for dyslexics compared to non-dyslexics the

median value was 10 prism diopters for dyslexics versus 17 prism diopters for non-dyslexics

In contrast convergence amplitude was in the normal range for the two populations while for

dyslexics the amplitude was significantly larger than for non-dyslexics the median value was

30 prism diopters and 18 prism diopters for dyslexic and non-dyslexic children respectively

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups

The mean fusional convergence capacities at distance were 1680 prism diopters for both the

control and the dyslexic group At near the corresponding figures were 2640 prism diopters

and 267 prism diopters respectively The mean fusional divergence capacity at distance was

650 and 620 prism dopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 105 and 102 prism diopters respectively There was no

statistically significant difference between the groups

210 Ocular Pathology

Ocular pathologies do not seem to be prevalent in the dyslexic population and it was not

reported on in the literature reviewed However according to Evans and Drasdo100 the

perspective to relate ocular pathology to dyslexia is in individuals who were normal readers but

developed reading disability due to pathological conditions such as stroke or tumors affecting

the right hemisphere These conditions may be relevant to the considerations of establishing the

relationship between ophthalmic problems and dyslexia100

69

Only one study83 included this as part of the investigation Hoffman83 reported that one child

(093) of the population of learning disabled children examined had acute conjunctivitis

A major study on dyslexic subjects (although outside the cope of the present study but worth

citing) was conducted by Raju11 in 1997 She studied oculomotor control in dyslexia and

reported that sixty five percent of the normal readers exhibited normal oculomotor control as

compared to the eleven percent of dyslexics and that fifty four percent of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills whilst twelve

percent of the control subjects displayed the same

70

LITERATURE REVIEW TABLE pages 70-72

71

72

73

211 Summary

Research into visual aspects of dyslexia has been undertaken mainly by optometrists and

ophthalmologists The main area of controversy is that ophthalmologists tend to deny any

relationship between vision and reading ability while optometrists assert that identifying and

correcting visual defects are an important part of managing dyslexia The findings of studies that

have investigated vision function in dyslexic population since the 1940s are inconclusive and

reflect the complexities associated with identifying the causes of dyslexic as well as its diagnosis

treatment and management

A range of study designs were used and the inclusion of a control group for example depended

on the objective of the study Some authors either compared the prevalence of vision defects in a

population of reading disabled group to the prevalence of similar vision defects in a group of

normal readers or only presented the prevalence of a particular visual condition in a group of

reading disabled children All the studies used convenience sampling methods at schools or by

including learners who were referred to their private practices the size being determined by the

number of dyslexic students available The main areas of vision functions studied were visual

acuity refraction binocular vision and ocular pathology The study findings were inconclusive

and the only vision variables consistently report ed across the studies was the association of

hyperopia with poor reading performance

The differences in results reported by the authors may be due to the methodological problems

such as differences in instrumentation and techniques failurecut-off criteria method of data

analysis lack of comparison group and subjectsrsquo selection from clinical population There was a

noticeable absence of studies on African children research has shown that African persons are

less at risk of myopia and more at risk for open-angle glaucoma than Caucasians There is

therefore scope for continued research among African children to determine whether there is any

relationship between dyslexia and vision

Research on dyslexia and vision that requires an interdisciplinary approach may yield different

results due to different approaches by different professionals However it is challenging to

74

design a study that would control for all variables possibly influencing reading ability and vision

function Reading performance is an extremely complex task and its measurement can be

influenced by many factors

Having reviewed the relevant literature on dyslexia and vision Chapter three will present the

research design the study population sampling procedure methods of data collection and

analysis

75

CHAPTER THREE METHODOLOGY

31 Introduction

This chapter will present the research design the study population sampling procedure

methods of data collection as well as method of data analysis

32 Research Design

The study was designed to provide empirical information to enable a comparison of the visual

characteristics of dyslexic children and normal readers A matched paired case-control

method was adopted Data was collected from both the dyslexic and control groups and

analysed

321 Sampling Design

Participants for both samples were selected using the convenient sampling method (based on

available subjects) and an optometric eye examination was conducted

322 Study Population and Participants

The study population comprised children attending a school for children with learning

difficulties from which the dyslexic children were selected to participate in the study while

the control group consisted of learners from a mainstream Durban school

i Dyslexic Group Participants for the study from the dyslexic group consisted of 31

African dyslexic children (the experimental group) selected from Khulangolwazi Special

School for children with learning difficulties in Clairwood South of Durban Psycho-

educational evaluationdiagnosis of dyslexia was not part of this study and was not

76

performed The school principal allowed access to all learnersrsquo file from which the

dyslexic learners where selected based on the school psychologistrsquos diagnosis Learners

attending this special school were referred from mainsream schools around Durban The

final diagnosis as to the type of learning difficulties was done by the school psychologist

None of the children were on any medication

The mean age for the dyslexic participants was 13 years and the learners were in grades

four through grade seven considering the fact that they were children that were reading two

grades behind their chronological age There were 15 boys and 16 girls Due to limited

number of learners who were classified as dyslexic it was difficult to recruit the targeted

number of one hundred participants for the study The subjects from both groups were

lsquoAfricanrsquo South Africans of African origin

ii Control Group Participants for the control group consisted of 31 children from a

mainstream school in Durban (Addington Primary School) The mean age of the children

was 11 years and 9 months A total of 15 boys and 16 girls were chosen ranging from grade

four to grade seven

323 Inclusion Criteria for the Dyslexic group

To be included in the study the subjects had to meet the following criteria

1 An average or above average intelligence quotient For this study an average was taken

as between 95 and above as recorded on the learners file by psychologist

2 Be two grades or more below grade equivalent in a mainstream school

3 The child has not been absent from school for more than 10 of the attendance days

The information on the learnersrsquo attendance was supplied by the school principal

324 Exclusion Criteria for the Dyslexic group

The following criteria resulted in children being excluded from the study

1 Presence of any emotional problems (information from the learnersrsquo file)

2 Presence of any systemic condition

77

3 Use of any systemic medication

The inclusion and exclusion criteria for the control group were similar to the experimental

group except that the children did not have any reading problems and were attending a

mainstream school

33 Ethical Clearance

The study protocol was approved by the then University of Durban-Westville research ethics

committee A letter of request for access to the schools was sent to the Department of

Education (Appendix B) Written informed consent (Appendix C) for access to the schools (for

both groups) was obtained from the Department of Education Written informed consents were

also obtained from the principals of the schools (mainstream and special school) (Appendix D)

Due to the difficulty in reaching the parents the school principals consented on behalf of the

parents for the learners to participate in the study The participants were fully informed of the

purpose of the study and accepted to co-operate with eye examination procedures

34 Testing Protocol

The testing was done first with the dyslexic learners and then at the mainstream school

35 Study Setting

The same testing arrangement was used for both groups A testing room was set up in one of

the offices provided by the school principal The distance acuity chart was placed at a point

six meters away from where the subjects sat and was used for the distance acuity chart and all

distance testing The examinations were done under room illumination except for some

procedures such as retinoscopy that required dim illumination resulting in the room being

dimmed

78

All testing was conducted in the morning as it was anticipated that better responses could be

obtained when the children were not tired Each school principal provided an assistant who

helped with various activities such as controlling room illumination The rationale and

technique for every procedure was fully explained to each participant and a trial reading was

taken to ensure that all instructions given were understood During the tests subjects were

asked (from time to time) if they were tired For any answer to the affirmative the testing was

discontinued and the child was allowed to have a break As a routine a break of 10 minutes

throughout the duration of entire procedure was allowed

The testing instruments were provided by the International Center for Eye Care Education

(ICEE) As the instruments were also being used by other researchers they were unavailable

on certain days resulting in some tests being postponed until a later date The data collection

took an average of two months per school All data were collected by the examiner alone and

each examination took an average of thirty minutes to complete

36 Data Collection Techniques

All tests were conducted in free space The data collection techniques used in the present

study was similar to techniques used in other studies 93100106-140 based on pediatric

populations (Table 31)

Table 31 Data Collection Tools and Measurement Outcome

InstrumentationTechnique Measurement Outcome

Bailey ndashLovie LogMAR Charts Visual Acuity

Royal Air Force (RAF) Rule NPC Amp of Accommodation

Welch Allyn Streak Retinoscope Objective Refraction

Trial Frame and Trial lenses Subjective Refraction

Cover Test - Occluder and Prism Bar Ocular Alignment

Maddox Rodpen torchPrism bar Heterophoria and Tropia

Maddox Wing Near Phoria

Prism Bar Heterophoria and Tropia

79

Pen torch PD rule Cornea reflex test-strabismus

Retinoscope and neutralizing lenses Accommodation Posture

Jackson cross cylinder Refine subjective findings

Trial Frame and neutralizing lenses Relative accommodation

+-2 flipper lenses Accommodation Facility

Direct Ophthalmoscope (Welch Allyn) Ocular Health

Prism Bars Fusional Reserves

361 Visual Acuity

Visual acuity is a measure of the acuteness or clearness of vision It assesses the ability of the

visual system to resolve detail which is dependent on the sharpness of the retinal focus within the

eye the sensitivity of the nervous elements and the interpretative faculty of the brain 106 Visual

acuity was assessed using the Logrithm of Minimun Angle of Resolution (LogMar) chart which

means that the letters change in size from line to line in equal steps of the log of the minimum

angle of resolution (MAR) Visual acuity was assessed monocularly and binocularly for both

distance and near The LogMar chart facilitates algebraic operations for statistical analysis and

allows for precise quantitative assessment of visual acuity It is recommended for use in research

studies in which visual acuity is a dependent variable106

Each subject was comfortably seated and the test was conducted with the examiner sitting in front

in such a way that the subjectrsquos view was not obstructed For monocular visual acuity the

subject was instructed to cover one eye with their right palm and to read the letters on the chart

This procedure was repeated binocularly and the test was done under normal room illumination 106 No subjects wore spectacles so visual acuity for aided vision was not indicated

362 Refractive Error

Refractive error was assessed objectively using the Streak Retinoscope (Welch Allyn) with a

+150D fogging lens (for an armrsquos length of approximately 67cm) with the subject fixating a

660 (to maintain fixation) optotype on the distance visual acuity chart As the subject focused

80

on the distance target on the chart the retinoscope light was passed across the subjects eye

(right hand used to scope right eye and left hand used to scope left eye) The motions

observed were neutralized with the appropriate lenses plus lenses for lsquowith motion and minus

lenses for against motion The subjective refraction was performed through monocular

fogging refined using the Jacksonrsquos cross cylinder followed by binocular balancing 106 The

retinoscopic results were used for analysis

Although retinoscopy was performed without cycloplegia as in other studies 395107108

cycloplegic refraction was not performed because the researcher (an optometrist) was not

licensed to use therapeutic drugs when the data was collected However it has to be

acknowledged that a non-cycloplegic examination among children has limitations among

those with hyperopia as the full extent of the hyperopia may be masked

363 Near Point of Convergence

The NPC was measured using the Royal Air force (RAF) rule The subject was instructed as

follows ldquoThis test measures your ability to turn your eyes in towards your nose Look

directly at the dot on middle of the line The image may appear to be blurring (not clear)

That is okay However if the dot becomes two say lsquotworsquo I will then pull the target away If

the dot becomes one again say lsquoonersquo rdquo The subject was asked to fixate on the dot on the

middle of the line as the target was advanced towards him The objective reading was taken at

the point when one eye loses fixation while the subjective reading was noted when the subject

reported the target to be double At a point when the subject reported double the target was

moved back until a point when the target was reported to be single again this was taken as the

recovery point The final break and recovery points recorded were the average of three tests

measurements The measurement was taken three times in order to detect fatigue which may

indicate poor convergence 111-113 The objective reading was taken for analysis

364 Strabismus

Strabismus was assessed using the Hirschberg test A penlight was held about 50cm from the

childs face in the mid plane and they were encouraged to fixate the light with both eyes open

81

The location of the corneal reflection on each cornea relative to the centre of the pupil was

then noted The displacement of the corneal reflection from the centre of the pupil (angle

lambda) was estimated in millimeters An ocular alignment was observed as a symmetric

displacement of the reflexes 114115 The examiner proceeded with the Kappa Test which is a

test of monocular fixation and is done while the child was monocularly viewing the examiners

penlight The penlight was maintained at 50 cm distance from the face The purpose of this

test was to determine the visual axis of each eye The examiner observed the location of the

corneal reflexes relative to the center of the pupil in the right eye and then in the left eye A

nasal displacement is signified by a (+) sign and a temporal displacement is signified by a (-)

sign with the amount quantified in millimeters away from center Again a normal location

will likely be either centered or +05 mm nasally displaced 114115

The interpretation of the magnitude and the type of deviation requires the data gathered from

both the Hirschberg tests The fixating eye is the eye in which the corneal reflex location is

the same on the Hirschberg and Kappa tests The strabismic eye is the eye in which there was

a difference The change in location from the Kappa tests as compared to the Hirschberg test

equals the amount and type of deviation Every 10 mm of corneal displacement is

approximately equivalent to 22 prism diopters A nasal change in location on Hirschberg

testing from the visual axis denoted by the Kappa test denotes an exotropia A temporal

change in location on Hirschberg testing from the visual axis denoted by the Kappa test

denotes an esotropia 114

365 Heterophoria

Ocular alignment was assessed using the cover test at six meters and 40cm for distance and

near respectively It is an objective technique and is very suitable for assessing ocular

alignment in children

For distance cover test the test target was a letter from the line above the subjectrsquos best visual

acuity of the worst eye The subject was advised that the muscle balance of their eyes was

assessed and that he should look steadily at the fixation target (an isolated letter on the chart)

at all times and ignore the occluder The coveruncover (unilateral) cover test was performed

82

to determine the presence of phoria or tropia 106114-115 To perform the unilateral cover test

the occluder was placed in front of the right eye held for approximately one to two seconds

and the left eye was observed for any movement If no tropia or phoria present the left eye

was covered and the right eye then observed Any movement observed was neutralized using

a loose prism bar in the corresponding base direction Base-in prism for exo deviation and

base-out for eso deviation

The alternating cover test was performed to assess the direction and magnitude of phoria or

tropia (it measures the total angle of deviation) 106114-115 The test conditions were the same as

the unilateral cover test The subject was comfortably seated and instructed to look at the

fixation target at distance The occluder was placed in front of the patientrsquos right eye held for

2-3 seconds and then quickly placed on the left eye The test was repeated several times with

the occluder held in front of each eye before quickly moving to the other eye while observing

the eye that was just uncovered Any deviation was neutralized accordingly 115 The Maddox

Wing was also used to assess near phoria under normal room illumination The technique was

assessed with the subject wearing his full subjective correction 115 The use of cover test for

the assessment of ocular alignment at both distance and near may have allowed for continuity

and standardization However the cover test and the Maddox wing were used by Evans et al 100 in the same study The subject was directed to look through the horizontal slits to view the

chart that comprised of a horizontal and vertical scale and a horizontal and vertical arrow The

construct of the Maddox wing is such that the right eye sees only the arrows while the left eye

sees only the scale It achieves dissociation by presenting independent objects to the patient

As the images are dissimilar the incentive to fusion is abolished and the eyes adopt the fusion

free position 115 The arrows are positioned at zero on the scales but through dissociation any

phoria will be indicated by an apparent movement of the arrow along the scale

To measure horizontal phoria the subject was asked ldquowhich white number does the white

arrow point tordquo The number on the scale represents the magnitude of the deviation and the

direction To measure vertical phoria the subject was asked to indicate which red number the

red arrow points to 115

83

366 Evaluation of Accommodation Functions

a Accommodation Posture The assessment of accommodation accuracy was performed at

the subjects habitual near distance using the monocular estimation method (MEM) The

MEM technique uses a specially made target with pictures or words suitable to the childs

reading level The card was attached to the retinoscope The procedure was performed

over subjects distance prescription and at his normal reading distance with enough light

to provide a comfortable reading vision Retinoscopy was performed on axis (right eye

before left eye) as the child reads the words or describes the picture on the card aloud As

the retinoscopic reflexes were observed an estimate of the magnitude of the motion was

made and trial lenses were then interposed briefly (approximately 05 seconds) to avoid

changing the childs accommodation status until no motion was observed The lens

power at which the motion was neutralized was taken as the lag or lead of

accommodation It is a lag of accommodation when the reflex is neutralized with

positive lenses while it is called lead of accommodation if neutralized with negative

lenses 112113116117

b Accommodation Facility this assesses the rate at which accommodation can be

stimulated and relaxed repeatedly during a specific time period It relates to the

individualrsquos ability to shift focus quickly and efficiently for varying distance The

procedure was performed with the best subjective correction in place monocularly and

binocularly but only the result for the binocular assessment was utilized for the data

analysis According to Wick and Hall 120 binocular accommodative facility testing may

not be a true accommodative facility measure such as monocular facility but clinically

binocular testing may be more useful because it gives a ldquoreal liferdquo assessment of

accommodation and binocular interactions of accommodation and vergence

The target used was letters on a 69 range on a near point card and at the subjectrsquos

habitual reading distance A lens flipper of 2 diopter lenses was used Letters and

pictures used were appropriate to the childs ability The subject was informed that he

would be shown two different lenses and that one pair causes the system to work while

84

the other pair relaxes it and was instructed to look at the materials as the lenses were

flipped Each time the print become clear and single he was to immediately report

clear The lenses were flipped and child should report clear when the material gets

clear To ensure that subject understood the instructions trial readings were first taken

The number of cycles completed per minute was recorded One cycle meaning clearing

both the plus and minus lens sides The test was done binocularly112-113118-120 At near

with no suppression control

c Amplitude of Accommodation The amplitude of accommodation was assessed by

Donderrsquos push ndash up method using a Royal Airforce (RAF) near point rule a rod with a

movable target and metrics as well as dioptric marking This procedure was performed

monocular and binocularly The subject was seated comfortably and instructed to read a

line of letters on the card that corresponded to a visual acuity of 69 and told to keep the

letters clear The target was slowly moved towards the child along the rule until the child

reported first sustained blur at which point the dioptric result was read off the Royal air

force rule Three readings were performed and an average taken This test was

conducted monocularly for each eye and then binocularly

As reported by Wick and Hall 120 the minimum amplitude of accommodation expected is

based on the subjects age and can be estimated from the Hofsetterrsquos formula

Minimum =15 - 025 (age)

Expected =185 - 03 (age)

Maximum = 25 - 04 (age)

The amplitude of accommodation was generally considered clinically important only

when it falls below the expected age minimum 120

d Relative Accommodation (RA) Relative accommodation tests are plus-to-blur (negative

relative accommodation) (NRA) and the minus-to-blur (positive relative accommodation)

(PRA) The relative accommodation tests assess patientsrsquo ability to increase and decrease

accommodation under binocular conditions when the total convergence demand is constant 106 It is also an indirect assessment of the vergence system since the vergence demand

85

remains constant while accommodative demand varies 121122

Relative accommodation was measured using the plus and minus lenses binocularly at

near with distance subjective results in place 122 In this procedure the child viewed a

reading target size letter size or picture of 66 size at the reading distance with the best

subjective correction in place Plus or minus lense powers was added in 025 diopter

steps in approximately 2 seconds interval121 to assess negative and positive

accommodation respectively This test was performed binocularly and the reading was

taken when the subject reported a sustained blur The relative accommodation is difficult

to assess with trial lenses121 The trial lenses were used to assess relative accommodation

in the present study The use of the phoropter was not possible considering the study

setting

367 Fusional Reserves

The term fusional reserve is used synonymously with vergence reserves and prism vergence

vergence amplitude 121123 or fusional amplitudes 111 Vergence ability is measured using

prisms placed in front of each eye The amount of prism is increased gradually to measure the

amount of fusional reserve the individual has in reserve to compensate for a phoria112 124 The

amount of base out prism required to produce diplopia is called positive fusional reserves or

positive fusional vergence while the amount of base in prism required to produce diplopia is

called the negative fusional reserves or negative fusional vergence 115

Positive fusional vergence (convergence) and negative fusional vergence (divergence) were

assessed using a prism bar in steps without suppression control although it has been reported

by Wesson et al 125 that when suppression is controlled the average vergence values will be

lower because the test is stopped when the suppression is detected that is if suppression is not

monitored the break is not detected until the stimulus is outside the suppression zone and a

higher vergence value may be obtained However

Scheiman et al 126 argued that the significance of such suppression in binocular individuals is

unknown

86

Assessing fusional reserves in children with loose prism bars may be a better technique

because it offers the possibility of viewing the eye movement objectively Secondly it

removes the restriction imposed by testing children behind the phoropter Finally the test

itself nearly duplicates the movement of the eyes under normal conditions allowing

convergence or divergence in discrete steps rather than as a smooth-pursuit type vergence

movement 125

The test was performed with the target at six meters (test target 69 letter line) and at 40 cm

(N5 text) The prism was placed in front of the right eye first Base-in was routinely assessed

first the rational for this sequence of testing being that the convergence response stimulated

during the base-out (or convergence) measurements may produce vergence adaptation ( a

fusional after-effect) which may temporarily bias the subsequent base in values in the base-out

direction 127 Typically there is no blur point for base -in vergence testing at distance This is

because at distance accommodation is already at a minimum and cannot relax beyond this

point 115

Base out prisms were used to measure positive fusional vergence while base in prisms were

used to measure negative vergence The subject was seated comfortably and instructed as

follows ldquoI am measuring the ability of your ayes to converge and diverge Watch the row of

letters Tell me when they first get blurry if they do Also tell me when they first become

two rows of letters and when they become one againPrime The prism was introduced over the

right eye lsquoDoublersquo was first demonstrated to the child by placing a 20 diopter prism over one

eye If blur was reported the prism power was increased until a break was reported The

prism power was then reduced until the subject reported that the rows of letters were single

The technique was then repeated for base out prism and then the test was repeated at near The

break and recovery points were determined subjectively from the childrsquos report of blur break

and recovery and objectively by observing the subjects eye movements The objective

findings were used for analysis

87

368 Ocular Health

The only test performed to evaluate ocular health was direct ophthalmoscopy

369 Vergence Facility

It was impossible to perform the vergence facility testing as the children could not fuse the

base in prisms So vergence facility data was not available for analysis The recovery point

of the base in reserves may provide useful information on the vergence facility According to

Bishop 111 ldquoblur and break points tend to reflect the quantity of fusion whereas the recovery

point indicates the quality of fusion that is the ease of change of fusional demands (facility)

and the ability to maintain fusion (stamina)

37 Statistical Analysis

The data entry was done by a staff optometrist who is skilled in statistical analysis The data

analysis was undertaken by the International Center for Eye Care Education statistician Data

was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 For comparison all data from both groups was subjected to a two-sample t-test (2-

tailed) (Table 32) Participants who did not meet the pass criteria for any of the test variables

were referred to their optometrist for further evaluation

Table 32 The Diagnostic Criteria for Each Test Variable

Variable Diagnostic Criteria

Visual Acuity 128-131 69

88

Refractive Errors 128-131

Hyperopia ge +075

Myopia ge -050

Astigmatism ge -075

Anisometropia ge 075 between the two eyes

Emmetropia lt - 050DS lt +100DS lt -075DCyl

Near Point of Convergence 132-134 ge 10cm

Accuracy of Accommodation 116117119120

Lag ge +075 Lead any minus finding

Accommodative Infacility 118119 gt 7cycles per minute in response to +2 -200 flipper testing

binocularly at near

Amplitude of Accommodation106119120

Binocular accommodative amplitude ge 2D below the expected

value for the patients age for minimum amplitude (using

Hofsetterrsquos formula) (15-14age)

Positive Relative Accommodation 106121122124

gt - 237DS

Negative Relative Accommodation 106121122124

gt + 2 DS

Phoria 106128

Distance

Near

gt 6 prism diopters exophoria or 4 prism diopters esophoria

gt 6 prism diopters exophoria or 4 prism diopters esophoria

Strabismus 131135 Manifest or intermittent manifest deviation of gt 2 prism

dioptres 135 or asymmetry in Purkinje reflex or compensating

fixation a misalignment of ge 25 degrees 131

Fusional Reserves LimitsDistance 119

Near 106121126

Base in Blur X Break 6 Recovery 4

Base out Blur 10 Break 16 Recovery 10

Base in Blur 14 4 Break12 5 Recovery 7 4

Base out Blur 22 8 Break 23 8 Recovery 16 6

89

38 SUMMARY

The study consisted of two groups of 31 participants from two schools in Durban The study

group was children with learning difficulties from which the dyslexic children were selected

and the control group was selected from a mainstream school The mean age for the dyslexic

participants were 13 years while the mean age of the children from the control group was 11

years and 9 months A convenience sampling method was adopted to select the study

participants due to the limited number of subjects to select from

The vision functions investigated may be divided into three broad areas of visual acuity and

refraction binocular vision and ocular pathology which corresponds with areas of vision

function investigated in the literature review Standard optometric testing procedures were

used to assess the vision variables

Visual acuity was assessed using the LogMAR chart at both distance and near Refractive

error was determined objectively using a streak retinoscope (without cycloplegia) while the

child fixated a 660 optotype at 6 meters with a fogging lens to control accommodation

The near point of convergence was measured using the Royal Air force rule (RAF) Ocular

alignment was assessed using the cover test at six meters and 40cm for distance and near

respectively For distance cover test the test target was a letter from the line above the

subjectrsquos best visual acuity of the worse eye The coveruncover (unilateral) cover test was

performed to determine the presence of a phoria or tropia The alternating cover test was

performed to assess the direction and magnitude of the phoria or tropia The test conditions

were the same as the unilateral cover test The Maddox Wing was also used to assess near

phoria under normal room illumination Strabismus was assessed using the Hirschberg test

with a penlight held about 50cm from the childs face in the mid plane and was encouraged to

fixate the light with both eyes open The location of the corneal reflection on each cornea

relative to the centre of the pupil was then noted

The different aspects of accommodation functions were also measured The assessment of

accommodation accuracy was performed at the subjects habitual near distance using the

90

monocular estimation method (MEM) The accommodation facility was assessed using a 2

diopter lens flipper with the best subjective correction in place The target used was letters on

a 69 range on a near point card and at the subjectrsquos habitual reading distance The amplitude

of accommodation was assessed by Donderrsquos push ndash up method using a Royal Airforce (RAF)

near point rule This procedure was performed monocular and binocularly with a 69 visual

acuity as the test target The relative accommodation was measured using the plus and minus

lenses The test target is a reading letter target size or picture of 66 size at the reading distance

with best the subjective correction in place This test was performed binocularly and the

reading was taken when the subject reported a sustained blur

The fusional reserves were assessed using prism bars in steps without suppression control

The test was performed at 6 meters (test target 69 letter line) and at 40 cm (N5 text) Base out

prisms were used to measure positive fusional vergence while base in prisms were used to

measure negative vergence Ocular health evaluation was assessed using the direct

ophthalmoscope

Data was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 To compare the data from both groups all data was subjected to a two sample t-test

(2-tailed) The methods used in this study were the same as those used for similar tests in the

international literature reviewed in Chapter Two and the sample size was larger than most of

the studies presented

Chapter Four will outline the study results with the comparative and descriptive statistical

analysis

91

CHAPTER FOUR RESULTS

41 Introduction

This chapter presents the study findings and the analysis of results obtained The data for the

prevalence of visual acuity refractive errors near point of convergence accommodation

functions and heterophoria are presented in histogram while the descriptive statistics are

presented in tables

42 Prevalence of Vision Defects

The prevalence is an indication of the subjects that have either achieved or did not achieve the

pass fail criteria and is not a normal distribution of a particular condition The prevalence rate

of the vision functions is expressed in percentages The other data is expressed as mean (M)

and standard deviation (SD) and calculated for descriptive purposes at 95 confidence

interval (CI)

If an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants would be expected

421 Visual Acuity (VA)

i Dyslexic Group In total 32 of subjects in the dyslexic group had visual acuity

less than 69 while 68 had visual acuity of 66 and 65 The distribution of those

subjects with VA le 69 is as follows 65 (2 subjects) had VAs of 69 65 (2

subjects) had VA of 612 65 (2 subjects) had VA of 615 3 (1 subject) had

VA of 630 3 (1 subject) each had VA of 638 648 660 respectively (Fig

41)

ii Control Group In total 32 of subjects in the control group had visual acuity

92

less than 69 while 68 had visual acuity of 66 The distribution is as follows 3

(1 subject) each had VA of 69 612 615 624 while 65 (2 subjects) had VA

of 630 638 and 648 respectively (Fig 41)

Fig 41 Prevalence of Visual Acuity

The mean visual acuity for the right eye for the dyslexic group was 017 031 and 000

024 for the right eye for the control group There was no statistically significant difference

between the two groups (p = 029) (Table 41) The mean visual acuity for the left eye for the

dyslexic group was 020 033 and 000 024 for the control group There was no

statistically significant difference between the groups (p = 023) (Table 41) LogMAR acuity

of 020 is equivalent to 69 in Snellenrsquos notation (Appendix E Visual acuity conversion table)

Table 41 Descriptive Statistics for Visual Acuity Variable Group N ( 95 CI) Minimum Maximum p

Mean (LogMAR)

SD

VAcuityRE

LE

DyslexicControl

DyslexicControl

3131

3131

017000

020000

031024

033024

000000

000000

010090

100090

029

023

422 Refractive Error

i Dyslexic Group Twenty three percent (23) of the dyslexic subjects had

refractive errors while seventy seven percent (77) were emmetropic (defined as

93

retinosocpic findings of less than 05 myopia less than +075 hyperopia or less than

075 astigmatism) (Fig 42) The distribution is as follows 65 (2 subjects) had

myopia 65 (2 subjects) had hyperopia (both latent) As cycloplegia was not

used in assessing refractive error an estimate of the diagnosis of latent hyperopia

was made based on a high difference between the retinoscopic findings and the

subjective refraction results In the event that the increased retinoscopic finding

(that is more plus) did not blur the subjectsrsquo vision latent hyperopia was assumed

About 10 (3 subjects) had hyperopic astigmatism 65 (2 subjects) had

amblyopia (defined as visual acuity in either eye of 2040 or a two-line difference

in acuity between the two eyes with no improvement with pinhole) 141 and 65 (2

subjects) had anisometropia (defined as a difference of ge 075 in sphere or cylinder

between the two eyes) (Fig 43)

ii Control Group Twenty two and a half percent (225) of the participants from

the control group had refractive errors classified as follows 65 (2 subjects)

had myopia 3 (1 subject) had hyperopia and 13 (4 subjects) had astigmatism

and 65 (2 subjects) had anisometropia No participant had amblyopia

Fig 42 Prevalence of Refractive Errors (total

01020304050607080

Myopia HyperopiaAstimatism (Hyperopic)Anisometropia Amblyopia Emmetropia

Prevalence

Dyslexic GroupControl Group

0

5

10

15

20

25

Prevalence

Refractive Errors (total)

Dyslexic GroupControl Group

94

Fig 43 Prevalence of the Types of Refractive Errors

Refraction data for two children from the dyslexic group who had cataracts could not be

obtained due to poor reflexes and were excluded from the analysis

The mean spherical equivalent (SE) refraction for the right eye for the dyslexic group was 086

098 and 070 103 for the control group was There was no statistically significant

differences between the two groups (p=066) The mean spherical equivalent refraction of the

left eye for the dyslexic group was 057 101 and 049 109 for the control group There

was no statistically significant difference between the two groups (p= 092) (Table 42)

Table 42 Descriptive Statistics for Refractive Error Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

RE (SE)

LE (SE)

DyslexicControl

DyslexicControl

2931

2931

086070

057049

098103

101109

-125-350

-150-350

500350

400350

066

092

423 Near Point of Convergence

i Dyslexic Group Thirty three percent (33 10 subjects) had NPC break points

(objective) of greater than or equal to 10cm while about sixty seven percent 67

(20 subjects) had NPCs of less than 10cm (Fig 44) One child complained of being

tired and did not participate in this procedure and was therefore excluded

ii Control Group Forty eight percent (48 15 subjects) had NPC break point

(objective) of greater than or equal to 10cm Fifty two percent (16 subjects) had

95

NPC less than 10cm (Fig 44) Data on NPC for one child from the dyslexic group

was lost

The mean NPC break was 890 503 for the dyslexic group and 1260 870 for the control

There was a statistically significant difference between the two groups (p = 0049) The mean

NPC recovery was 1400 588 for the dyslexics and 2200 820 for the control group

There was a statistically significant difference between the two groups (p = 006) (Table 43)

Table 43 Descriptive Statistics for NPC Break and Recovery Points Variable Group N (95 CI) Minimum Maximum P

Mean SD

NPC Break

NPC Recovery

DyslexicControl

DyslexicControl

3030

3030

8901260

14002200

503870

588820

500400

600800

26003400

28003800

049

006

424 Ocular Health

Six and a half percent (2 subjects) in the dyslexic group had cataracts (unclassified) on both

eyes No pathology was detected in the control group

425 Heterophoria

i Dyslexic Group No subject in the dyslexic group manifested with a phoria at

96

distance

At near 3 (1 subject) had an esophoria greater than or equal to 4 prism diopters

3 (1 subject) had an esophoria of 3 prism diopters 65 (2 subjects) had

exophorias of greater than or equal to 4 prism diopters 3 (1 subject) had an

exophoria greater than or equal to 8 prism dioptres and 65 (2 subjects) had

exophorias of 6 prism diopters (Fig 45)

ii Control Group At both distance and near no subject in the control group had

phoria of greater than 2 prism diopters

Fig 45 Prevalence of Phoria at Near (40cm) with Maddox Wing

Four children from each group could not complete the test as they left to attend lessons The

mean exophoria for the dyslexic group was 163 261 and 180 042 for the control group

There was no statistically significant difference between the two groups (p = 059) The

mean esophoria for the dyslexic group was 350 070 and 2 000 for the control group

There was no statistically significant difference between the two groups (p= 046) (Table 44)

No tropias were observed

Table 44 Descriptive Statistics for HeterophoriaVariable Group N ( 95 CI) Min Maxi P

97

Mean SD

At NearExophoria

Esophoria

DyslexicControl

DyslexicControl

2727

2727

163180

350200

261042

070000

000100

300200

1000200

400200

059

0469

426 Accommodation Functions

a Accommodation Posture As shown in Figure 46 the distribution of accommodation

lag is as follows

i Dyslexic Group

Thirty nine percent (11 subjects) had a lag of accommodation (defined as + 075 or

greater) three and a half percent (1 subject) had lead of accommodation (defined as

- 025 or greater) while fifty seven percent had normal accommodation posture

ii Control Group

Forty two percent (13 subjects) had lag of accommodation About eleven percent

(11) (3 subjects) had lead of accommodation while about thirty nine percent had

normal accommodation posture

Three children from the dyslexic group could not continue with this test as they had to

leave to attend class activities while three children from the control group sought

permission to be out of the testing room at that point These children were excluded

from the analysis for binocular accommodation lag

98

Fig 46 Prevalence of Accommodation Lag ( ge+075)

The mean accommodation lag for the right eye was 091 038 for the dyslexic group

and 092 057 for the right eye for the control group There was no statistically

significant differences between the two groups (p =083) The mean for the left eye for

the dyslexic group was 085 036 and 091 048 for the control group There was no

statistically significant difference between the two groups (p = 061) (Table 45)

Table 45 Descriptive Statistics for Accommodative PostureVariable Group N (95 CI) Minimum Maximum P

Mean SD

Accomm lag

RE

LE

DyslexicControl

Dyslexic Control

2828

2828

091092

085091

038057

036048

000-050

-050-050

200200

125200

083

061

b Accommodative Facility As shown in Figure 47 the distribution of accommodation

facility was as follows

i Dyslexic Group

Forty six percent (46 13 subjects) had normal accommodative facility (defined as

0

10

20

30

40

50

Prevalence

Bi no cu lar A ccom m od ation Lag

D y s lex ic G r o u p

C on t ro l G ro u p

99

greater than 7cpm) binocularly at near while (54) (15 subjects) had

accommodation facility less than or equal to 7cpm (Fig 47)

ii Control Group

Sixty seven percent (67) (18 subjects) had normal accommodative facility

(defined as greater than 7cpm) while thirty three percent (33) (9 subjects) had

accommodation facility less than 7cpm

Three children from the dyslexic group and four from the control did not complete the test

as they indicated that they were tired The mean binocular accommodation facility was

686 cpm 274 for the dyslexic group and 885 369 for the control group There was a

statistically significant difference between the two groups (p=0027) (Table 46)

Table 46 Descriptive Statistics for Binocular Accommodative FacilityVariable Group N ( 95 CI ) Minimum Maximum p

Mean SDAccommodativeFacility at near

DyslexicBE

ControlBE

28

27

686

885

274

369

2

2

12

21

002

7

c Amplitude of Accommodation Monocular amplitude of accommodation in both eyes

and for both groups did not differ significantly (not more than 050D) except for one

subject who had a difference of 4 diopters between the two eyes The amplitude of

100

accommodation for two subjects who had latent hyperopia in the dyslexic group was

relatively low (600DS and 800DS) as compared to age minimum amplitude of

accommodation of 1175DS The amplitude of accommodation for two participants who

had cataracts were excluded from the analysis

Only the data for the monocular amplitude was analyzed When accommodation

amplitude is assessed monocularly it measures the response for each eye individually

and is particularly important to determine whether a patient has accommodative

insufficiency 133 The monocular amplitude of accommodation has been used to assess

amplitude of accommodation function in several studies 86 88100102-103 The mean for the

right eye for the dyslexic group was 1198 234 and 1287 108 for the control group

There was no statistically significant difference between the two groups (p =007) (Table

47) The mean for the left eye for the dyslexic group was 1214 214 and 1287 116

for the control group There was no statistically significant difference between the two

groups (p =022) (Table 47)

Table 47 Descriptive Statistics for Amplitude of Accommodation Variable Eye Group N ( 95 CI ) Minimum Maximum p

Mean SDAmplit of Acomm

RE

LE

DyslexicControl

DyslexicControl

2931

2931

11981287

12141287

234108

215116

810

810

2015

2015

007

022

d Relative Accommodation (NRA PRA) As shown in Fig 48 the results for the relative

accommodation were as follows

For the PRA all subjects in both groups had PRAs of more than -200DS For the

negative relative accommodation (NRA) 89 of the dyslexic subjects had NRA greater

than +200DS while 96 of subjects from the control group had NRA greater than

+200DS

101

Fig 48 Prevalence of Relative Accommodation (NRA+2PRA-200)

Three children from the dyslexic group and five from the control group could not

continue with the test for the assessment of relative accommodation as they had to attend

important class activities

The mean PRA for the dyslexic group was -623 117 and -606 063 for the control

group There was no statistically significant difference between the two groups (p

=051) The mean NRA for the dyslexic group was 322 079 and 311 047 for the

control group There was no statistically significant difference between the two groups

(p =068)(Table 48) The findings for RA were higher than the published norm This

may be due to the process of addition of trial lenses

Table 48 Descriptive Statistics for Relative Accommodation Variable Group N ( 95 CI ) Minimum Maximum p

Mean SDPRA

N RA

DyslexicControl

DyslexicControl

2826

2826

-623-606

322311

117063

079047

-900-700

200200

-400-500

60045

051

068

427 Fusional Reserves

The children in both groups either could not report or understand blur so the result for break

102

and recovery was used in all analysis of vergence function

a Base-in Vergence Reserves at Distance two children from the dyslexic group could not

complete all aspects of the fusional reserves assessment because they were tired These

children were excluded in the analysis

The mean base-in to break for the dyslexic group was 1469 683 and 1600 350 for

the control group There was no statistically significant difference between the two

groups (p =046) The mean base in to recovery for the dyslexics was 1172 620 and

1280 317 for the control group There was no statistically significant difference

between the two groups (p =049) (Table 49)

Table 49 Descriptive Statistics for Base-in Vergence Reserves at distanceVariable Group N (95 CI) Minimum Maximum P

Mean SD

BI to Break (dist)

BI to Recovery (distance)

DyslexicControl

DyslexicsControl

2931

2931

14691600

11721280

683350

620317

410

28

4022

3520

046

049

b Base-in Vergence Reserve at Near The mean base in to break at near was 1185 514

for the dyslexics and 1283 313 for the control group There was no statistically

significant difference between the two groups (p=029) The mean base in to recovery was

872 478 for the dyslexics and 1032 335 for the control group There were no

statistically significant differences between the two groups (p=017) (Table 410)

Table 410 Descriptive Statistics for Base-in Vergence Reserves for NearVariable Group N (95 CI) Minimum Maximum P

Mean SDBI to Break (near)

BI to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

11851283

8721032

514313

478335

26

14

2518

2015

029

017

103

c Base-out (BO) Vergence Reserves at Distance Four children from the control

group could not give a report on the recovery point Two children from the dyslexic group

could not complete the test The mean base out to break at distance for the dyslexic group

was 2706 925 and 2416 975 for the control group There was no statistically

significant difference between the two groups (p=024) The mean BO to recovery for the

dyslexic group was 1876 796 and 17 693 for the control group There was no

statistically significant difference between the two groups (p= 040) (Table 411)

Table 411 Descriptive Statistics for Base-out Vergence Reserves at Distance Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

BO to Break(Distance)

BO to Recovery(dist)

DyslexicControl

DyslexicsControl

2931

2927

27062416

18761700

925975

796693

10001000

400600

0004000

35003500

024

040

d Base-out Vergence Reserves at Near The mean base-out to break at near was 2160

1162 for the dyslexic group and 2109 842 for the control group There was no

statistically significant difference between the two groups (p =084) The mean base-out

to recovery at near for the dyslexic group was 1335 745 and 1555 625 for the

control group There was no statistically significant difference between the two groups

(p =016) (Table 412) The comparative and descriptive statistsics for all variables is

shown in Table 413

Table 412 Descriptive statistics for base-out vergences at near Variable Group N (95 CI) Minimum Maximum P

Mean SD

BO to Break (near)

BO to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

21602109

13351555

1162842

745625

8001000

600800

4040

3530

084

016

104

Table 413 Comparative Descriptive Statistics for Dyslexic and Control groups

Variable Dyslexics( 95 CI)

Control( 95 CI)

N Mean SD N Mean SD PVisual Acuity

RE 31 017 031 31 000 024 029LE 31 020 033 31 000 024 023

Refractive ErrorRE 29 086 098 31 070 103 066LE 29 057 101 31 049 109 009

NPC Break 30 890 503 31 1260 870 049NPC Recovery 30 14 588 31 2200 820 006

Heterophoria (Near)Exo 27 163 261 27 180 042 059Eso 27 350 070 27 200 000 046

PRA 28 -623 117 26 -606 063 051NRA 28 322 079 26 311 047 068

Amp of AccommodationRE 29 1198 234 31 1287 108 007LE 29 1214 215 31 1287 116 022

Accom Facility (bin) 28 686 274 27 885 369 003

Accomm LagRE 28 091 038 31 092 057 083LE 28 085 036 31 091 048 061

BI Break (Dist) 29 1469 683 31 1600 350 046BI Recovery (Dist) 29 1172 620 31 1280 317 049

BI Break (Near) 29 1185 514 31 1283 313 029BI Recovery (Near) 29 872 478 31 1032 335 017

BO Break (Distance) 29 2706 925 31 2416 975 024BO Recovery (Distance) 29 1876 796 31 1700 693 040

BO Break (Near) 29 2160 1162 31 2109 842 084BO Recovery (Near) 29 1335 7 45 31 1555 625 016

105

Summary Statistical tests were performed on the data to ensure scientific validity and were presented for

comparative and descriptive purposes The prevalence of visual acuity was similar between the

dyslexic and control groups and there was no statistically significant difference between the two

groups (p = 029 right eye 023 left eye)

The prevalence of total refractive errors was similar between the two groups and there was no

statistically significant difference between the two groups (p = 066 right eye and 092 left

eye) Hyperopia was slightly more prevalent in the dyslexic group than in the control group

The prevalence of myopia was the same in both groups Astigmatism was more prevalent in

the control group than in the dyslexic group The prevalence of anisometropia was the same in

both groups Amblyopia was more prevalent in the dyslexic group compared to the control

group

The control group had a higher near point of convergence break and recovery points than the

dyslexic group The NPC showed a statistically significant difference between the two

groups NPC break (p =0049) and NPC recovery (p =0006) The prevalence of exophoria at

near was higher in the dyslexic than the control group There was no statistically significant

difference between the two groups (p= 059) The prevalence of esophoria in the dyslexic

group was higher than that found in the control group but there was no statistically significant

difference (p =046)

The prevalence of relative accommodation was similar in both groups and there was no

statistically significant difference between the groups The dyslexic group had a reduced

amplitude of accommodation compared to the control group and there was no statistically

significant difference between the two groups (p=007 right eye andp = 022 left eye) The

dyslexic subjects performed significantly worse than the control subjects in accommodative

facility function and there was a significant difference between the two groups (p =0027)

The dyslexic subjects had a lower accommodation lag than the subjects from the control

group There were no statistically significant differences between the two groups right eye (p

= 083) and left eye (p = 060)

At near the negative fusional vergence (base in vergences)) was reduced in the dyslexics

compared to the control group while the positive fusional vergence (base out vergences) was

similar for both groups There was no statistically significant difference between the groups

106

(p = 021 for BI break at near) and (p = 084 for BO break at near) Having analyzed the

results Chapter Five will discuss the findings in the context of the international studies

reviewed in the literature

107

CHAPTER FIVE DISCUSSION

51 Introduction

A review of the literature reveals that studies 2356878990 conducted on dyslexic children have

investigated the prevalence of vision defects in the dyslexic population and examined whether

such vision anomalies are correlated with dyslexia by comparing vision characteristics in

dyslexic subjects to normal readers

The aim of this study was to determine the prevalence of vision defects in a South African

population of the dyslexic school children and investigate the relationship between vision and

dyslexia by comparing the vision characteristics of the dyslexic to the control group of learners

from a mainstream school The proposed null hypothesis was that there was no statistically

significant difference between the means of the vision functions in the dyslexic population

compared to the control group A discussion of the results of this study is presented

52 Prevalence of Vision Defects

521 Visual Acuity

The 32 prevalence of visual acuity worse than 69 was the same for both groups A reduced

visual acuity could be due to ocular abnormality (such as cataract) or due to normal refractive

error differences in the population Visual acuity defects due to ocular diseases are usually not

common in paediatric populations142 but tend to be related to refractive error changes in the

population143 This relates more in the present study as the prevalence of defects of visual acuity

and the total refractive errors were similar Consequently in relating visual acuity to refractive

errors myopia astigmatism and hyperopia may reduce distance or near visual acuity

108

Visual acuity testing is widely performed in screening refraction and monitoring of disease

progression It is also used in both basic and clinical vision research as a way to characterize

participants visual resolving capacities144 Specific to the present study visual acuity testing

helps detect visual problems at the distances at which most school learning activities occur and

gives useful preliminary information on how good or poor a childrsquos vision is reduced distance or

near visual acuity gives an indication of poor vision that will necessitate further examination

However in assessing how visual acuity relates to reading performance it is important to note

that visual acuity as measured with the traditional letter identification only assesses the ability to

discern letters on the visual acuity charts and does not assess dynamic vision behaviour

binocular eye control or ocular stress and so may not necessarily mean normal vision

Furthermore normal visual acuity may not necessarily mean normal vision since some people

may have other vision defects such as color vision or reduced contrast or inability to track fast-

moving objects and still have normal visual acuity The reason visual acuity is very widely used

is that it is a test that corresponds very well with the normal daily activities a person can handle

and evaluates their impairment to do them 145

In relation to previous studies the visual acuity findings in this study are similar to reports by

Buzzelli 6 Goulandris et al 90 and Metsing and Ferreira 103 On the contrary Hoffman 83 found

a lower prevalence (1028) of visual acuity worse than 69 in a group of learning disabled

children In contrast to the present study however the mean age of subjects in Hoffmanrsquos study

was 8 years 6 months while the mean age in the present study was 13 for dyslexics and 11years 9

months for the control group Grnlund et al 140 documented that age must be taken into

consideration when describing and comparing VA in different populationsbecause VA develops

from birth to adolescence A lower prevalence (10) of visual acuity worse than 69 was also

reported by Sherman84 Latvala et al 3 reported a slightly worse distance acuity in the dyslexic

group than in the control group but noted no statistically significant difference Evans et al 5

found that dyslexic groups had a significantly worse visual acuity than the control group

(Binocular distance VA p =00164 and binocular near VA p =00018) Similarly Ygge et al2

reported that the subjects from the control group had a better visual acuity than the dyslexic

group at both distance and near and that the results showed statistically significant differences

(distance at p =003) (near at p=0005) Bucci et al 89 and Kapoula et al 87 reported the

109

prevalence of visual acuity of all participants in their study to be normal A study conducted by

Grisham et al 86 lacked a control group but it was reported that 568 of the participants had 66

acuity in both eyes (compared to 68 in dyslexic group in the present study) 172 had 69 or

worse (compared to a total of 315 who had visual acuity of 69 and worse in the present study)

The mean age of the participants in the study by Grisham et al 86 was 154 years while the mean

age of the participants in the dyslexic group in the present study was 13 years

The factors which may affect the outcome of visual acuity measurements that may lead to

variations in results reported by different authors include size position and distribution of retinal

mosaics (retinal photoreceptors) 143 optical aberration pupil size clarity of the optical media

age method and type of chart used contrast isolated or multiple letters state of accommodation

illumination as well as the criteria used to define visual acuity114 140 Psychological factors

affecting visual acuity testing results include blur interpretation fatigue and malingering 143

These factors may be difficult to control and could constitute confounding variables which may

lead to variations in visual acuity results reported by different authors

522 Refractive Error

The prevalence of uncorrected refractive error of 23 found in the dyslexic group was similar to

the 225 found in the control group and there was no statistically significant difference between

both groups (p = 066 right eye p = 092 left eye) This finding may mean that the dyslexic

subjects are not at more risk of a particular refractive anomaly compared to participants from the

control group as the dyslelxic gourp has similar distribution of refractive errors

Some studies 5838498103146 did not classify refractive error according to the types but presented

the results for the total refractive error On this basis the prevalence of total refractive error

(23 for dyslexic and 225 for the control group) in the present study is similar to reports by

other studies Hoffman83 reported a 215 prevalence of refractive error in a study of the

incidence of vision difficulties in learning disabled children Although there was a marked

difference in the sample size in both studies the similarities in the findings may be because as

with the present study Hoffmans study was based on a population of learners in a special school

110

referred for optometric care by educators psychologists and reading specialists

Similar to the present study Evans et al 5 and Ygge et al 2 reported that the prevalence of

refractive errors in the dyslexic and control groups was similar Metsing and Ferreira103 found

no statistically significant difference between the learning disabled and the mainstream

groups in their study Contrasting findings were reported by Sucher and Stewart 98 who found

the prevalence of refractive errors to be 94 for the control group although the 208 found in

their dyslexic group is similar to the 23 found in the present study Wesson 146 reported the

prevalence of refractive error in children with reading dysfunction to be 38 while the

prevalence of refractive errors was 16 in the study by Sherman 84 In the study by Alvarez and

Puell 88 the prevalence of refractive error was 424 in the dyslexic group while 415 of the

control group had refractive error When compared to the present study the higher prevalence of

refractive error in the dyslexic group can be accounted for by the relatively high prevalence of

hyperopia (287) and in the control group the higher prevalence of refractive error can be

accounted for by the high prevalence of myopia (194) It is unclear why the prevalence of

hyperopia was as high as no information was given on how refractive error was assessed

Statistically for the present study the mean refractive errors for the dyslexic group were right

eye 086 098 left eye 057 1013 and right eye 070 103 left eye 049 109 for the

control group There was no statistically significant difference (RE p =066 LE p = 092)

Evans et al 91 reported a mean spherical refractive error of 077DS (right and left eye similar) In

the study by Alvarez and Puell 88 the refractive error results (descriptive statistics) reported

were as follows for the poor readers mean spherical equivalent refractive errors for the right and

left eye were 020 06 and 020 06 respectively In the control group mean spherical

equivalent refractive errors for the right and left eye were - 020 08 and - 014 08

respectively The descriptive statistics on refractive errors was not reported in the studies 3 93

cited earlier which made it impossible to compare the statistical findings with the present study

In the present study about 68 of participants from both groups were emmetropic (defined as lt -

050DS +100DS and 075D cyl) while 655 of the dyslexic children and 645 of children

from the control group were emmetropic in the study by Alvarez and Puell 88 Only the study by

111

Alvarez and Puell 88 reported on emmetropia

Refractive errors are among the leading causes of visual impairment worldwide and are

responsible for high rates of visual impairment and blindness in certain areas School children

are considered a high risk group because uncorrected refractive errors can affect their learning

abilities as well as their physical and mental development147

Several factors may lead to variations in results found in different studies This include the type

of population studied (clinical or non-clinical) sampling method (convenience or cluster)

classification criteria examiner bias and more specifically the use of cycloplegia in the

assessment of refractive errors

The following studies2 3 8895 categorized refractive errors as hyperopia myopia astigmatism

anisometropia or amblyopia

a Hyperopia In the present study hyperopia appeared slightly more prevalent (65) among

the dyslexic group than in the control group (3) This result agrees with the report from

other studies 93 95 and Alvarez and Puell88 who reported a much higher prevalence (287)

for the dyslexic group compared to 161 of the control group Helveston85 found no

difference in the prevalence of hyperopia between the population of normal and poor

readers

Hyperopia is the refractive error that is consistently reported 3939598 to be associated with

reading difficulties so it was expected that one will find the higher prevalence of hyperopia

in the present study

In the present study two children had latent hyperopia (based on the assumption that

increased plus did not blur the distance vision) The full magnitude of the hyperopic

findings could not be estimated because cycloplegia was not used Therefore it is possible

that the prevalence of hyperopia may have been under-estimated Williams et al 148 studied

the ldquoprevalence and risk factors for common vision problems in childrenrdquo and concluded

112

that the 48 prevalence of hyperopia in their study was ldquoan underestimate as data was

collected without cycloplegiardquo Borghi and Rouse (cited by Scharre and Cree)149

determined that on an average 063D more plus was identified by cycloplegic refraction

In relating hyperopia to reading difficulties simple to moderate hyperopia may not cause

constant blur at a distance or near point but the extra accommodative effort produces

asthenopic symptoms of intermittent blur headache fatigue and loss of concentration and

inattention in some patients which may be mistaken for short attention span Uncorrected

hyperopia is associated with esophoria at near point which can stress the fusional vergence

systems that hold the eyes in correct alignment If the hyperopia and esophoria is excessive

an accommodative esotropia can result 81150 This may result in difficulty in reading

b Myopia The 65 prevalence of myopia was the same in both groups This result is similar

to an equal prevalance of myopia reported by Eames 95 (4 in both groups) Alvarez and

Puell 88 reported that 57 of dyslexics were myopic while 194 of the control group was

myopic The prevalence of myopia was higher in studies by Rosner and Rosner 93 (54

non- learning disabled and 19 learning disabled) and lower than the prevalence reported

by Grisham and Simons150 In contrast to the present study subjects from the study by

Rosner and Rosner 93 were patients of an eye clinic where the two main reasons for referral

were reduced visual acuity and school learning problems while subjects for the present

study were referred to the special school mainly due to low academic performance which

may not necessarily be vision- related The subjects were chosen for the study irrespective

of whether they complained of a visual impairment or not whereas in the study by Rosner

and Rosner 93 study subjects had been identified as having a visual impairment Therefore

the higher prevalence may have been from the selection protocol used by Rosner and

Rosner 93 in their study

The prevalence of myopia was not reported in several studies 3568498146 referenced but was

presented as ldquorefractive errorsrdquo (was not classified as myopia hyperopia or astigmatism)

The onset and progression of myopia may be influenced by factors such as environment

nutrition genetic predisposition premature and low birth weight the effect of close work

113

racial and cultural factors differences in pupil size and illumination143151 Variations in the

prevalence of myopia reported by different authors may be related to the variation in the

above-mentioned factors

c Astigmatism

The prevalence (13) of astigmatism in the control group was higher than in the dyslexic

group (10) and there was no statistically significant difference between the two groups

Similarly Alvarez and Puell 88 found a higher prevalence of astigmatism in the control

group compared to the dyslexic group astigmatism was detected in 161 from the control

group compared to 8 and 92 right and left eyes respectively from the dyslexic group

In contrast Latvala et al 3 reported that 36 in the dyslexic group had astigmatism and

none from the control group had astigmatism Ygge et al 2 reported that the prevalence of

astigmatism was higher in the dyslexic group (28 left eye 25 ) compared to the control

groups (right eye 183 left eye 243) although there was no statistically significant

difference between the groups (p=025)

Rosner and Rosner93 reported that 30 of non-learning disabled and 27 of learning

disabled participants had astigmatism which differed markedly from the findings in the

present study despite the similarities in the study population Eames95 found an equal

prevalence of astigmatism (7) between the two groups studied

Astigmatism affects vision in different ways Clinically astigmatism over 150 diopters can

often cause severe eye strain and interfere with reading and even lesser degrees of

astigmatism can be symptomatic in some patients 81

Naidoo et al 157 studied refractive error and visual impairment in African school children in

South Africa Although the study was conducted specifically on mainstream school

children it relates to the present study as it provided useful information on the visual

characteristics of school children in South Africa and forms a basis for comparison since all

the studies reviewed in the present study were conducted on Caucasian population except

114

for the study by Metsing and Ferreira 103 which did not provide much information on

refractive errors in the their study of learning disabled children in Johannesburg South

Africa

Naidoo et al 157 reported a prevalence of hyperopia of 18 myopia of 29 astigmatism

of 67 and 68 right eye and left eye respectively In comparison the prevalence of

refractive error for the control group in the present study was hyperopia 3 myopia 65

and astigmatism 13 The refractive error trend (astigmatism being higher in prevalence

than myopia and hyperopia) was similar between the present study and the study by Naidoo

et al 157 despite the difference in prevalence reported Given that ethnic origins culture

socio-economic class are comparable between both studies the difference in prevalence

may (partly) have been due to different sampling methods (cluster versus convenience) and

the use of cycloplegia in their study The use of cycloplegia has been reported to yield

spherical aberrations and unpredictable errors due to associated mydriasis 110

It has also been documented 158 that the use of cycloplegia yields more positive retinoscopic

findings although this did not seem to apply in this comparison as the prevalence of

hyperopia in the present study was higher than that reported by Naidoo et al 157 A possible

explanation for the difference in prevalence may be related to the different criteria used to

define hyperopia Bennet et al 159 stated that the degree of ametropia ldquohellip considered to be

within normal limits in a particular study will drastically affect the prevalencerdquo Similar

opinion was noted by Gronlund et al 140 who stated that ldquothe choice of definition will of

course have a great impact on prevalence In their study on Swedish children aged 4-15

years Gronlund et al 140 defined astigmatism as ge 075 and stated that the prevalence of

astigmatism would have been 21 (instead of 32) had they defined astigmatism as ge

100 Furthermore Grisham et al 86 in their ldquoStudy of poor readers in high schoolrdquo noted

that helliprdquoif one selected less stringent criteria then the proportion of students with any given

dysfunction would decreaserdquo In the present study the criteria used to classify hyperopia

were ge +100D while Naidoo et al 157 used ge +200DS cut- off

115

d Anisometropia Anisometropia may be defined as a difference of 075 (sphere or cyl) or

more between the two eyes and this definition was the criteria used in this study 129

Anisometropia is of great clinical interest because of its intimate association with

strabismus and amblyopia141 152

The 65 prevalence of anisometropia was similar in both groups This is comparable to

results reported by Latvala et al 3 where the prevalence of anisometropia was 36 (2

subjects) for the dyslexic group and 6 (3 subjects) for the control group but contrasts with

findings by Eames 95 who reported the prevalence of anisometropia to be 13 for poor

readers and 6 for the control group while Ygge et al 2 found a higher prevalence of

anisometropia in the control group (158) than in the dyslexic group (94)

In anisometropia the difference in refraction as well as the refractive error causes the image

to be out of focus on one retina blunting the development of the visual pathway in the

affected eye 141 The fovea of an anisometropic eye receives images from the same visual

object however the images from the more myopic or hyperopic eye are out of focus 153 It

appears that anisometropia is a major cause of amblyopia for at least one third of all

amblyopias 131 and anisometropia is considered to be a causal factor in the pathogenesis of

amblyopia and strabismus It is also estimated that 6 to 38 of all cases of amblyopia are

caused by anisometropia without strabismus whereas in about 12 to 18 of the children

with strabismus this is accompanied by anisometropia 154

According to Grosvenor 114 uncorrected anisometropia even of a moderate amount may

induce eyestrain because it is impossible for the accommodation mechanism to maintain

clear images on the retina at the same time On the other hand large amounts of

anisometropia (2DS or more) may not cause symptoms as no effort is made to maintain

single binocular vision 114

Rutstein and Corliss155 investigated the relationship between anisometropia amblyopia and

binocularity and concluded that higher degrees of anisometropia generally cause deeper

116

amblyopia and poorer levels of binocularity for hyperopes but not for myopes and stated

that ldquoAlthough uncommon small amounts of anisometropia can cause moderate

amblyopiardquo However the results of the present study failed to support this claims as none

of the subjects had amblyopia resulting from anisometropia

In anisometropia the displacement or distortion of the image prevents the development of

fine visual perception in the occipital cortex and puts the child at risk for developing

amblyopia 141 Anisometropia causes poor reading skills probably through the mechanism

of poor sensory and motor fusion rather than reduced visual acuity It degrades binocular

coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 150

e Amblyopia Two subjects (65) in the dyslexic group had ambyopia due to cataracts and

none from the control group had amblyopia This result may be comparable to studies by

Latvala et al 3 who reported 36 (2 subjects) for the dyslexic group and that of Rosner and

Rosner 93 with a prevalence of 4 for the non-learning disabled group and 3 for the

learning disabled group Cataract is a major cause of amblyopia (deprivation ambyopia) by

causing an impediment to the visual axis 141 Other conditions that may lead to amblyopia

include strabismus (which causes the images on the retinas to be dissimilar) and uncorrected

refractive errors (especially as in anisometropia) Anisometropia is related to amblyopia

Amblyopia is a major public health problem It is the most common cause of monocular

vision loss in children and young adults Early recognition and prompt referral are crucial

especially during infancy and childhood to prevent permanent loss of

vision 141

523 Heterophoria

Heterophoria is the relative deviations of the visual axes when the eyes are dissociated 160 Some

degrees of heterophoria are considered normal for persons with normal binocular vision

Approximately 1-2 prism dioptres of esophoria or 1- 4 diopters of exophoria at distance should

117

be considered to be within normal limits161 At near 3-6 prism dioptres of exophoria is

considered normal (physiological exophoria) 161 A normal healthy eye is usually able to

overcome these small deviations and so it is described as being compensated161 However

esophoria is much less compatible with comfortable vision than is exophoria Any amount of

esophoria in a patient with visually related symptoms may be problematic 162

In the present study all subjects were orthophoric at distance This report corroborates reports by

other authors Mathebula et al 163 studied heterophoria in a South African population of school

children (mean age 10 16 years) and reported that the mean distance horizontal heterophoria

showed orthophoria Similarly Evans et al 100 reported that all subjects examined ldquowere

orthophoric at distance with cover testrdquo According to Dowley160 ldquothe high prevalence of

orthophoria is realrdquo and that ldquothe high prevalence of distance orthophoriahellip may be due to the

coordinating influences of the same mechanisms that subserves prism adaptationrdquo A similar

view was expressed by Walline et al 164 who studied the ldquodevelopment of phoria in childrenrdquo and

stated that ldquonearly all of our subjects were orthophoric at distancerdquo

The major findings in heterophoria was a 95 prevalence of exophoria at near which was more

in the dyslexic subjects than in the control group with no exophoria There was no statistically

significant difference between the two groups (p= 059) At near 3 (1 subject) of the dyslexic

group had an esophoria greater than or equal to four prism diopters An interpretation of the

above finding is that the higher prevalance of heterophoria may mean that the dyslexic subjects

may be more uncomfortable when doing near work than the nroaml readers A possible

explanation may be that according to von Noorden (cited by Kommerell et al)165 heterophoria

typically causes asthenopia Patients with asthenopic symptoms (feeling of heaviness dryness

and soreness of the eyes pain in and around the eye frontal and occipital headache and the eyes

are easily fatigued) often have an aversion to reading and studying Typically such complaints

tend to be less severe or to disappear when patients do not use their eyes in close work165 It has

been documented that decompensated heterphoria (heterophoria accompanied by symptoms) can

be due to poor fusional reserves problems with sensory fusion (such as in anisometropia) or may

be due to an unusually large phoria which may be due to anatomical reasons or uncorrected

hyperopia 161166 The presence of phoria could contribute to visual and attention abnormalities

118

noted among children with reading difficulties making near point visual activities more

strenuous

Nearpoint esophoria has been reported to be associated with childhood myopia progression 164167

In the present study the only subject that had four prism diopters esophoria at near in the

dyslexic group had +125 hyperopia while the subject that had 3 prism diopters esophoria had -

150 DS myopia However due to the small sample size in the present study it is difficult to

draw any conclusion on the association between myopia and esophoria

The findings on heterophoria as in other aspects of vision functions in the dyslexic populations

reviewed are mixed However similar to the present study Latvala et al 3 assessed near

heterophoria using the Maddox Wing and reported a higher prevalence of near exophoria in the

dyslexics than in the control group Similarly Kiely et al 12 documented that dyslexics tend to

be more exophoric than normal readers Evans et al 101 found a higher prevalence of lateral

phoria in learning disabled subjects compared to normal readers In contrast to the above

findings Sucher and Stewart 98 assessed phoria using the cover test (testing distance was not

indicated in the results) but found a slightly higher prevalence of phoria in the control group than

in the dyslexics Evans et al 100 assessed near horizontal phoria using the Maddox Wing as in

present study but found no difference in near horizontal phoria between the dyslexic and control

groups Bucci et al 89 reported that there was no difference in phoria results at both distance and

near between the dyslexic and control groups Metsing and Ferreira 103 found no statistically

significant difference between the learning disabled and the mainstream groups on

heterophoriaThe use of the cover tests for both distance and near to assess heterophoria may have allowed for

standardization in technique but may not have implied that more valid results will be obtained

The cover test and the Maddox Wing have been used simultaneously to assess heterophoria in

other studies3 100 The possible sources of variations in results reported by different investigators

include classification criteria and poor technique 168

524 Near Point of Convergence

119

In the present study the prevalence of a remote (greater than 10cm) near point of convergence

was higher in the control group with 48 than in the dyslexic group with a prevalence of 33

There was a statistically significant difference in the near point of convergence break (p=0049)

and recovery (p = 0006) between the two groups

It may be that children who do not have reading difficulties tend to read more often than children

who experience difficulty to read This is because with increasing ability to read there is likely

to be more demand on accommodation and convergence resulting in near point stress as well as

esophoria The normal readers use and exercise the accommodation and convergence system

more frequently According to Owens and Wolf-Kelly 169 near work has long been considered a

potential source of visual problems stating that in their study reading ordinary text at a near

distance for about one hour induced significant changes in the resting postures of both

accommodation and vergences The authors indicated that ldquonear work induces a recession of the

near point of accommodation or vergencesrdquo

The near point of convergence break is associated with changes in inter-pupillary distance with

age As inter-pupillary distance widens with physical growth the amount of convergence in

prism diopters increases for a given distance from the subject 132 Chen et al 170 studied near point

of convergence in children aged 1-17 years and reported that an increasing incidence of remote

near point of convergence with increasing age in their study might be due to the near work

demands of primary school which might create a different level of near point stress than the near

work conditions in pre- primary school years

Similar to the findings in the present study Evans et al 91 reported a statistically significant

relationship between NPC and reading retardation (p=0019) Contrasting findings were reported

by other authors Bedwell et al 171 studied visual and ocular anomalies in relation to reading

difficulties and reported that 466 of the group of ldquogood readersrdquo had abnormal near point of

convergence compared to 58 of ldquopoor readersrdquo Similarly Bucci et al 89 and Kapoula et al 87

reported that the near point of convergence was significantly more remote in dyslexics than the

control In the study by Latvala et al 3 1275 of the dyslexic group and only 2 from the

120

control group had a near point of convergence of worse than 8cm A statistically significant

difference between the two groups was reported (p=00385) Metsing and Ferreira 103 found

no statistically significant difference between the learning disabled and the mainstream

groups on the near point of convergence function In a study that lacked control subjects

Grisham et al 86 reported a lower prevalence (154) of NPC (9cm or further) in a group of high

school poor readers The authors measured NPC three times on each subject in other to detect

fatigue but stated that it should therefore be noted that fatigue may be a problem for poor

readers even if they are able to converge at 8 cm if they tire easily then the ability to sustain

convergence during longer periods of reading may suffer

In the present study the measurement of the near point of convergence was repeated three times

on each subject in order to detect visual fatigue that the children experience everyday This

approach was documented by other authors 868789 111-113 Some authors111-113 have recommended

the measurement of the near point of convergence several times in order to detect fatigue which

is often indicative of poor convergence fusion system which may affect distract a child from

reading 172

525 Accommodation Functions

a Accommodation Facility Accommodative facility assesses the rate at which

accommodation can be stimulated and inhibited repeatedly during a specific time period It

relates to the individualrsquos ability to shift focus quickly and efficiently for varying distances

and is extensively used in the reading process 6

Only the result for the binocular accommodative facility was recorded and was used for analysis

in the present study Siderov and Johnston 174 noted that monocular accommodation

measurements provides a direct evaluation of the dynamics of accommodative response while

binocular testing of accommodative facility provides similar information but also reflects the

interactive nature of the relationship between accommodation and vergences

For the dyslexic group 54 (15 subjects) had inefficient accommodation facility while only 33

121

(9 subjects) of the control group had inefficient accommodative facility Statistically the control

group had a better accommodative facility (885cpm 369) than the dyslexics (686cpm 274)

and there was a statistically significant difference between the two groups (p =0027) It has

been documented 100 that an abnormal accommodative facility could imply difficulty in changing

focus from far to near and subsequently lead to a lack of interest in learning An efficient facility

of accommodation is particularly important because a greater period in school involves changing

focus between the chalkboard to near task such as writing at a desk Low values of

accommodative facility have been associated with symptoms related to near point asthenopia134

The result of the present study corresponds with findings by Evans et al 100 who reported that

dyslexics appeared to be slower at a test of accommodative facility Similar results were reported

in other studies 128898 but contrasts with results reported by Buzzelli 6 Buzzelli 6 assessed

accommodation facility using a Bernell Vectogram that utilized a polarised target to control for

suppression Buzzelli6 assessed accommodation facility as follows patients were asked to clear

the number 9 on the Bernell Vectogram target at 40cm They alternately viewed the target

through a 2D lens The test was continued for 20 cycles (a cycle being clearing the plus and

minus lens each presented once) The total time in seconds was recorded for the right eye left

eye and both eyes The polarized target controlled for suppression The use of the vectogram

to assess accommodation facility by Buzzelli6 differs from the flipper technique that was used in

the present study Secondly there was no control for suppression in the present study The

difference in results found between the two studies may be due to the lack of control for

suppression in the pre sent study Siderov and Johnston174 stated that ldquoclinical measurements of

binocular accommodation facility could vary depending on whether or not suppression has been

monitoredrdquo

A total of 317 of the poor readers in the study by Grisham et al 86 had inadequate (9cm or less)

accommodative facility

Allison173 suggested that there are different norms submitted by different authors for

accommodative facility measurements and that apart from adhering strictly to recommended

norms for accommodative facility testing other factors to be considered when evaluating

accommodative facility include the difficulty and speed of responses to the plus and minus lenses

122

and whether the subjects fatigue easily 173 Kedzia et al (cited in Zadnik)175 documented that

the factors that affect the testing of accommodative facility include the size and position of the

text being read the complexity of the target reaction time of the child to call out the symbol and

the magnification or minification factors introduced by the lenses themselves The above

variables are factors to be considered in assessing the variations in the findings for

accommodative facility reported in this study as compared to other studies

b Amplitude of Accommodation Accommodation is the ability to adjust the focus of the

eye by changing the shape of the crystalline lens to attain maximum sharpness of the retinal

image of an object of regard The absolute magnitude of the accommodative response is

termed the accommodative amplitude 120

Statistically the dyslexic group had a slightly reduced monocular amplitude of

accommodation (mean 12D) compared to the control group (1287D) but there was no

statistically significant difference between the two groups (p=070) The slight difference

in the mean amplitude of accommodation between the two groups may have been due to the

two subjects from the dyslexic group who had latent hyperopia

Similar to the present study lvarez and Puell88 reported that monocular accommodative

amplitude was significantly lower in the group of poor readers Evans et a l91100 in two

seperate studies 91100 reported that amplitude of accommodation was reduced in the

populations studied Ygge et al 102 and Goulandris et al 90 found no statistically significant

difference between the dyslexic and control groups Grisham et al 86 found that 247 of

the children had amplitude of accommodation which the authors classified as ldquoweakrdquo or

ldquovery weakrdquo while 115 had accommodative amplitudes classified as ldquoborderlinerdquo In the

present study the mean amplitude of accommodation for each group was within normal age

norms according to the amplitude norms and Hofsteterrsquos formula 106161

In the study by Metsing and Ferreira 103 a higher percentage of low amplitude of

accommodation for the right and left eyes respectively (516 and 533) was found in the

mainstream group compared to 291 and 288 in the learning disabled group The

123

relationships between the mainstream group and reduced amplitudes of accommodation of

the right (p=004) and left eyes (p = 0001) were found to be statistically significant

(plt005) Furthermore the relationship between the mainstream group and reduced ampli-

tude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to be moderate and

that between the mainstream group and the right eyes (Cramerrsquos = 0286) to be low

When accommodation amplitude is assessed monocularly it measures the response for each

eye individually and the limiting factor is the magnitude of blur-driven accommodation

Monocular amplitude measures are particularly important to determine whether a patient

has accommodative insufficiency 107 The amplitude measured binocularly is usually

greater than the monocularly measured amplitude due to the influence of both the blur-

driven accommodative response and accommodative and vergence responses 120 175

Clinically the amplitude of accommodation is considered important mainly when it falls

below the expected age norm in which case the child may experience blur vision at near

Secondly a difference of up to 2D between the two eyes may also be considered clinically

significant 120

An inefficient accommodation function may lead to difficulties in learning as the focusing

system of the eyes play a major role in the learning process Children who suffer some

anomalies of accommodation are more prone to fatigue quickly and become inattentive than

those who have norrmal accommodation function 87 Symptoms of accommodation

insufficiency are specifically related to near vision work 176

Factors affecting the measurement of amplitude include differences when measurements

are taken monocularly as compared to when assessed binocularly the angle of gaze target

size age refractive error race and climate 175

c Accommodation Posture During near vision the eyes are not usually precisely focussed

on the object of regard but the accommodation lags a small amount behind the target If

the accommodation lag is small then the blur it causes is insignificant if high then it can

result in blurred print during reading 142

124

About 3928 (11 subjects) of dyslexic subjects had lag of accommodation compared to

4193 (13 subjects) of the control group The mean accommodation lag was similar in

both groups 092 for control and 088 for the dyslexic group There were no statistically

significant differences between the two groups right eye (p = 083) and left eye (p = 060)

Similarly Evans at al 100 reported that the mean accommodation lag did not show any

statistically significant difference between the dyslexic and control groups On the

contrary Metsing and Ferreira 103 found a high prevalence of lead of accommodation for

the mainstream group but a high prevalence of lag of accommodation in the learning

disabled group The relationships were found to be statistically significant (plt005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000) In another

study Evans et al 91 reported a mean accommodation lag of + 112DS

An individual with a lag of accommodation habitually under accommodates and may lead

to difficulty with reading An accommodative response that manifests as an excessive lag

of accommodation may indicate latent hyperopia esophoria or may be associated with

accommodative insufficiency or accommodative spasm poor negative vergences or when

patient is overminused 177 The prevalence of high lag may be related to the prevalence of

latent hyperopia and esophoria in the present study

C Relative Accommodation (PRA NRA) The relative accommodation tests assess the

patientsrsquo ability to increase and decrease accommodation under binocular conditions when

the total convergence demand is constant67 It is also an indirect assessment of the

vergence system since the vergence demand remains constant while accommodative

demand varies 121 The results for the relative accommodation for all subjects from both

dyslexic and control groups were unexpectedly high The norm for the mean NRA and PRA

were given as approximately +2 or - 2 diopters from two reports consulted 119 121 while the

relative accommodation results in this study ranged between 3 and - 6 diopters Similarly a

higher relative accommodative value was reported by Chen et aland Abidin139 in a study of

vergence problems in Malay school children The only available study accessed that

125

reported on relative accommodation was conducted by lvarezand Puell 88 who reported

that the negative and positive relative accommodation values were similar in both groups of

children Statistically the results were (NRA Control group 19 06 poor readers 19

06 PRA Control group 23 09 Poor readers 20 13) compared to the findings from

the present study (NRA Control group was 311 047 NRA dyslexic group 322 079)

(PRA Control ndash 606 063 PRA dyslexic was -623 117) The p values were not

indicated

The higher values found in the present study may be due to the process of addition of trial

lenses specifically The phoropter has been reported to be a better technique to assess

relative accommodation 119

Although the relative accommodation was not assessed by many authors different opinions

were expressed regarding the assessment of relative accommodation Latvala et al 3 noted

that positive and negative relative accommodation ranges are ldquodifficult to take and

unreliablerdquo According to Morgan 178 the PRA and NRA results are often unusually high

but recommended that such tests results can be used as a sort of indicative way and can only

be interpreted loosely and suggested that the use should be restricted to special

circumstances Garcia and Fransisco 122 reported that high NRA can be associated with

disorders such as accommodative insufficiency and convergence excess while high values

of PRA are related to anomalies in which accommodative excess appears but concluded that

PRA and NRA findings were used mainly as complementary diagnostics tests of some

disorders in literatures consulted

A high value of the negative relative accommodation could indicate that the children were

exerting maximum accommodation effort Because 250D of accommodation is exerted at

the 40cm working distance the maximum amount of accommodation that would be

expected to relax accommodation at 40cm would be 250D A value greater than 250D

would mean that accommodation may not have been fully relaxed during the subjective

refraction 114 Generally a high value of the NRA may also mean that the refraction may

126

have been under corrected for hyperopia or over corrected for myopia 122 Garciaand

Francisco122 reported the relationship between high NRA values and under corrected

hyperopia to be due to the etiology of hyperopia

According to Grosvenor 114 the amplitude of accommodation is a limiting factor for the

expected value of the PRA For example ldquoIf a patient has an amplitude of accommodation

of only 150D we would expect the minus-lens-to-blur test to be no greater than -150Dhelliprdquo

A high value of the positive relative accommodation is related to convergence insufficiency 122 Illumination and the range of fusional vergence also affect the relative accommodation

An excessive illumination will give an erroneously high finding due to the increased depth

of focus 100

Dysfunctions of accommodation can significantly interfere with the comfort clarity speed

and accuracy of reading as the child develops reading skills 51 Wick and Hall 120 suggested

that the results for the tests of accommodation are more meaningful when analyzed

together as the results of individual accommodation function may not give a true reflection

of the childrsquos accommodation dysfunction

526 Fusional Reserves

Fusional vergence amplitudes is a clinical measurement of a person`s fusional vergence ability It

reflects the ability of the oculomotor system to maintain sensory fusion in spite of varying

vergence requirements 179 The horizontal vergence reserves describe the amount by which the

eyes can be converged or diverged whilst the subject maintains fusion 100161 It is a measure of

how much fusional vergence is available in reserve that can be used to overcome a phoria The

amount of base out prism required to produce diplopia is called positive fusional reserves

(measures convergence) 100 The measurements of convergence fusion are also referred to as the

lsquoamplitude of fusional reserves 111 Clinically the fusional amplitude provides information about

a patientrsquos ability to maintain comfortable binocular vision 100112

The participants from both the dyslexic and control groups either could not report or understand

127

blur so the result for break and recovery was used in all analysis of vergence function Evans et

al 100 reported similar observations For the fusional reserves only the findings for the near

positive fusional vergence (convergence) are emphasized The positive fusional

reservesvergence (measures convergence ability) may be more important in assessing reading

dysfunction More so it has been suggested by other authors 112121180 that near measurements

are more relevant in assessing vision functions in dyslexic children

Based on Bishops 111 recommended norm of between 30 and 40 prism diopters for positive

fusional amplitude at near the prevalence of abnormal positive fusional vergence at near was

83 for the control and 74 for the dyslexics This appears unusually high Walters 168

explained that a possible reason for an unexpected high base out finding is that exophoric

conditions makes compensating for base out more difficult than base in testing It is unclear if

this is the case in the present study although the prevalence of exophoria at near in the present

study was 95 The compensation for a phoria is maintained by fusion reflex so knowing the

magnitude of the vergence amplitude that is needed to compensate a given phoria is very useful

clinically181

For the present study at near the negative fusional vergence (base in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base out

vergences) was similar for both groups Overall there was no statistically significant difference

between the groups (p = 021 for BI break at near) and (p = 084 for BO break at near) Evans et

al 100 reported that the dyslexics have reduced fusional reserves compared to the control group

Bucci et al 89 reported reduced divergence capabilities in dyslexics at near

When assessing fusional reserves the blur point is a function of the flexibility between vergence

and accommodation When little flexibility exists the addition of even low prism causes a

simultaneous accommodation shift with resultant blur Lack of low blur findings may indicate an

inadequate flexibility bond between accommodation and convergence The break point is an

indication of the quality of binocular function Break values will be adequate when fusion ability

is good while the recovery measure is a more subtle indicator of the quality of binocular function

Consequently reduced blur break or recovery findings indicate the presence of near point stress

128

182 Some subjects from both groups had break values higher than 40 prism diopters for base in

and base out reserves Wesson et al 125 in their study reported a similar observation on objective

testing of vergence ranges Such high break findings may be because suppression was not

controlled while assessing the fusional reserves in the present study When suppression is

controlled the average vergence values will be lower because the test is stopped when the

suppression is detected If suppression is not monitored the break is not detected until the

stimulus is outside the suppression zone and a higher vergence value is obtained 125 However it

is important to note that the significance of such suppression in binocular individuals is unknown 126 Again in special cases such as suppression large abnormal findings (compared to

normative ranges) may permit better functions in reading than a lower abnormal finding 183 For

example an individual who totally suppresses the vision of one eye is less apt to have difficulty

at reading than the individual who only partially suppresses the vision of one eye 183

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups The

mean fusional convergence capacities at distance were 1680 prism dopters for both the control

and the dyslexic group At near the corresponding figures were 2640 prism diopters and 2670

prism diopters respectively The mean fusional divergence capacity at distance was 650 prism

diopeters and 620 prism diopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 1050 and 1020 prism diopters respectively There was no

statistically significant difference between the groups

Latvala et al 3 reported a prevalence of fusional amplitude greater than or equal to 32 prism

diopters to be 61 for the control group and 75 for the dyslexic group using referral criteria

of 32 prism diopters at a distance of 33cm At near a testing distance of 40cm was used in this

study The difference in prevalence between the two studies might be from the different test

distances used Furthermore as detailed by Rouse et al 158 large intra subject intra-examiner

and inter-examiner variations in the assessment of fusional reserves have been reported

Vergence amplitudes have been reported to vary with alertness that is whether the subject is

tired or rested or under the influence of a toxic agent 184

129

Bedwell et al 171 reported that neither convergence nor divergence were significantly related to

reading difficultyrdquo Similarly Metsing and Ferreira103 found no statistically significant

difference between fusional vergences amplitude (Base-out base-in) at near No details were

provided

There are several limitations with the assessment of fusional reserves Sheedy (cited by Rouse et

al) 158 remarked that ldquo the general opinion is that when fusional vergence tests are repeated on

the same patient the second value found may be quite different from the first a difference of 10

PD from one fusional vergence amplitude measurement to another is not unusual unless rigorous

controls ar a typical difference of 3- 4 prism diopters but can measure differences as large as 12

prism diopter on follow up visit and that inter-examiner variation can be as large as 10 to 16

prism diopters The authors concluded that the positive fusional vergences difference could be

due to children having difficulty understanding the instructions or expected endpoints children

being slower responders or may be poorer observers The blur readings on the vergence reserves

for both distance and near were excluded in the analysis as it was difficult to get the children to

elicit a proper response It is possible that the children did not understand the instruction or the

concept of blur despite repeated explanations and trial readings This variable is often difficult

for young subjects to understand Similar problems were encountered by Evans et al 100 who

commented that several subjects were unable to appreciate a blurrdquo Scheiman et al 126 stated

that ldquowe were unable to consistently obtain a blur finding from our samplerdquo and that rdquo less

than13 of the subjects were able to report a blur we therefore only recorded the break and

recovery findingsrdquo

Wesson et al125 emphasized that the ldquomajor limitations of any vergence measurement

isrdquohellipnecessity to obtain a subjective response to blur break and recovery and that working with

children whose responses are not reliable is difficultrdquo It was suggested that a break and recovery

point could be determined by observing the subjects eye movements as prism power is increased

This technique was applied in this study However most children have difficulty maintaining

fixation long enough to measure vergence ranges

Vergence is influenced by several factors including awareness of the distance of the object

130

(proximal vergence)cross-linking with the accommodative system (acommodative vergence) and

the fine tuning of ocular alignment during the fusion of each monocular image into a single

percept (fusional vergence) 161 and other factors such as test target and lighting conditions 125

Generally in the assessment of a patientrsquos binocular status the fusional reserves may be more

useful when analyzed with the phoria measurements 115

The vergence system is closely related to the accommodative system and symptoms may at times

appear similar The symptoms associated with deficiencies with the vergence system include

letters or words appear to float or move around postural changes noted when working at a desk

difficulty aligning columns of numbers intermittent diplopia at either distance or near 185

Furthermore ldquodysfunctional vergence could cause an excess of eye movements especially of

small saccades The ability to continue the rapid decoding of the visual characters decreases as

more stress is placed on the vergence mechanism of the dyslexic during reading which makes

them tire more quickly than normal readersrdquo 6

A possible explanation for the relationship between vergence control and reading proposed by

Riddel et al (cited by Morad et al) 186 is that children with poor vergence control had impaired

accuracy of spatial localization that may impend their ability to accurately determine the position

of letters within words

527 Ocular Pathology

Two participants from the dyslexic group had cataracts Only the study by Hoffamn 83 reported

that one child had conjunctivitis

54 Summary

The results of this study do not vary significantly from the international studies in spite of the

differences in study designs tools and sample sizes Although studies have indicated differences

131

in the prevalance of open angle glaucoma and myopia between Caucasian and African

populations this study does not indicate a specific difference in the prevalance of vision variables

between Caucasian and African dyslexic school children However the lack of studies on

African children makes it difficult to reach any conclusions regarding structural differences that

may affect the presence of dyslexia Comparing the findings of this study with those done

internationally highlights the complexity of investigating the visual conditions that may be

associated with dyslexia and the need for rigorous and consistent testing methods in order to be

able to compare results

Having completed a discussion of the study findings in Chapter Five Chapter Six will present the

study conclusion and indicate its limitation as well as recommendations for future studies

132

CHAPTER SIX CONCLUSION

61 Introduction

Being able to read and write is an essential part of being able to interact with the world As

vision plays a major role in reading and the learning process a normally working visual system is

essential for efficient reading

With as many as 20 of Caucasian children being affected to a greater or lesser degree by

dyslexia 1631 there is a need to identify its cause and develop remedial actions to reduce its

effects As studies 4 59 have shown that intellectual ability is not the cause of dyslexia research

to enable people to reach their full potential in spite of their impaired reading ability requires a

multidisciplinary approach of which optometrists are part

This study aimed to determine the prevalence of vision conditions in a South African population

of African dyslexic children and to study the relationship between vision and dyslexia in an

African setting This was done by investigating visual acuity refraction and binocular functions

between two groups of 31 African school children one group attending a school for children with

learning difficulties and the other a mainstream school This enabled a study of possible vision

defects in African dyslexic children as well as a comparison between the two groups

The study targeted African school children its objectives being to

1 determine the distribution of visual acuity disorders among dyslexic children

2 determine the distribution of refractive errors among dyslexic children

3 determine the distribution of heterophorias among dyslexic children

4 determine the distribution of strabismus among dyslexic children

5 determine the distribution of accommodation disorders among dyslexic children

6 determine the distribution of vergence disorders among dyslexic children

7 determine the distribution of ocular pathology among dyslexic population

133

8 compare these findings to a similar group of non-dyslexic children

A review of the literature showed that most studies that investigated visual functions in dyslexic

school children were conducted on Caucasian population and that the findings of these studies

were inconclusive The broad areas of vision investigated in the previous studies 2-6 87-89 were

visual acuity and refraction binocular vision and ocular pathology

As studies conducted on the African population was lacking this study examined visual acuity

(using LogMar charts) refraction (static retinoscopy without cycloplegia) near point of

convergence (RAF rule) [accommodation functions 2 flipper lenses to assess accommodation

facility MEM retinoscopy for accommodation posture and pushndashup method to evaluate

amplitude of accommodation] ocular alignment (cover test and Maddox Wing) and strabismus

(Hirschberg test) and fusional reserves (using prism bars)

The study was conducted on a African population of 31 dyslexic school children selected from a

school for children with learning difficulties and 31 control participants from a mainstream

school in Durban Their ages ranged between 10-15 years The participants were selected using

the convenient sampling method as there were only a few participants classified as being

dyslexics All data collection procedures were conducted at the respective schools

The prevalence of vision conditions were presented in percentages () Data was analyzed using

the Statistical Package for Social Sciences (SPSS) and was expressed as mean (M) and standard

deviation (SD) and calculated for descriptive purposes at 95 confidence interval (CI) The level

of significance considered to support a hypothesis was taken as P lt 005 For comparison all

data from both groups was subjected to a two sample t test (2-tailed)

134

62 Summary of findingsIf an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants (control group) would be

expected

621 Visual Acuity

The prevalence of visual acuity worse than 69 (23 dyslexic and 225 control) is the same in

both the dyslexic and the control group and there was no statistically significant difference (p =029 023 right and left eye respectively This is similar to the findings by Buzzelli 6 and

Goulandris et al 90 which found no statistically significant difference between the two groups

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexia and the control group (p=029 for the right eye and 023 for the left eye)

622 Refractive Errors

The prevalence of refractive errors was similar in both groups There was no statistically

significant difference between the two groups (p=066 for the right eye and 092 for the left eye)

This is similar to the findings by Evans et al 2 which found no statistically significant difference

between the two groups (p=058)

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 066 for the right eye and 092 for the left eye)

a Hyperopia Hyperopia was more prevalent in the dyslexic group than in the control group

This is consistent with findings in other studies 88 93 95

b Myopia The prevalence of myopia was the same in both groups This is similar to the

findings by Eames 95 which found no difference in the prevalence of myopia between the

two groups

135

c Astigmatism Astigmatism was more prevalent in the control group than in the dyslexic

group This is similar to the findings by Alvarez et al 88 which found a higher prevalence of

astigmatism in the control group than in the dyslexic group

d Anisometropia The prevalence of anisometropia was the same in both groups (2 subjects

each) This is comparable to results reported by Latvala et al 3 where the prevalence of

anisometropia was 36 (2 subjects) for the dyslexic group and 6 (3 subjects) for the

control group

e Amblyopia Amblyopia was more prevalent in the dyslexic group compared to the control

group This is similar to the findings by Latvala et al 3 and Rosner and Rosner 93 which

found a higher prevalence of astigmatism in the dyslexic than in the control group

623 Near Point of Convergence

The prevalence of remote NPC (ge10cm) was higher in the control group than in the dyslexic

group and there was a statistically significant difference between the two groups (p= 0049) This

is similar to the findings by Kapoula et al 87 Latvala et al 3 and Evans et al 91 who found a

statistically significant difference between the dyslexic and the control groups on the NPC

functions

The null hypothesis was rejected as there was a statistically significant difference between the

dyslexic and control groups (p = 0049)

624 Heterophoria

The prevalence of exophoria at near was higher in the dyslexic group (95) than in the control

group with no exophoria There was no statistically significant difference between the two

groups (p= 059) The prevalence of esophoria in the dyslexic group was higher than that found

136

in the control group and there was no statistically significant difference (p =046) This is similar

to findings in other studies 3 12101163 which found no statistically significant difference between

the two groups

The null hypothesis was accepted as there was no statistically significant difference between the

dyslexic and control groups (p=059 for exophoria) and (p=046 for esophoria)

625 Accommodation Functions

a Amplitude of Accommodation The dyslexic group had a reduced monocular amplitude

of accommodation compared to the control group but there was no statistically significant

difference between the two groups (right eye p = 007 left eye p=022) This is similar

to the findings reported in other studies 3 87 91102 which found no statistically significant

difference between the two groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation amplitude (right eye

p=007 left eye p=022)

b Accommodation Facility The participants from the dyslexic group performed

significantly worse than the control group in accommodative facility function and there

was a significant difference between the two groups (p=0027) This result corroborates

findings reported in other studies 12 88 98100 which found a statistically significant

difference between the two groups

The null hypothesis was rejected as there was a statistically significant difference between

the dyslexic and control group in accommodation facility (p=0027)

c Accommodation Lag The prevalence of lag of accommodation was higher in the control

group compared to the dyslexic group and there was no statistically significant difference

There were no statistically significant difference between the two groups (right eye p=083

137

and left eye p= 060) This is similar to the findings by Evans et al 100 which found no

statistically significant difference between the two groups The mean accommodation lag in

the present study were 092D for control and 088D for the dyslexic which is comparable to

reports by Evans et al 91 who reported a mean accommodation lag of + 112D

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation posture

d Relative Accommodation (PRA NRA) The mean values for the relative

accommodation are similar in both groups and there was no statistically significant

difference between the groups (p=068 for NRA and p =051 for PRA) This is similar

to the findings reported Alvarez and Puell 88 which found no statistically significant

difference between the two groups in relative accommodation

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups in relative accommodation

626 Fusional Reserves

For the present study at near the negative fusional vergence (base-in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base-out

vergences) was similar for both groups There was no statistically significant difference between

the groups (p= 021 for BI break at near) and (p = 084 for BO break at near) This is similar to

the findings by Evans et al100 which found no statistically significant difference between the two

groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 021 for BI break at near) and (p = 084 for BO break at near)

627 Ocular Pathology

138

Two participants from the dyslexic group had cataracts

63 Implications of the Study Findings

a Causal Relationship This study presented the prevalence of some vision defects in a

South African African population of dyslexic school children as compared to a population

of non-dyslexic children and could not establish any causal relationship between vision and

dyslexia The vision parameters that showed that statistical significance may only be

reflective of factors associated with dyslexia but in a non-causal way This may mean that

the prevalence of the particular visual variable is much higher in the dyslexic population

than in the control group Vision is essential for reading but the vision anomalies that are

found to be associated with dyslexia also occur in children who do not have reading

difficulties It is therefore possible that other complex vision functions may be more related

to the etiology of dyslexia than peripheral visual factors such as NPC and accommodative

facility which showed statistically significant differences in the present study

b Risk Identification The findings of this study also suggest that dyslexic children are not

at more risk for the identified vision condition compared to non-dyslexic children

However even when vision defects may not be the cause of dyslexia the presence of

uncompensated vision defect in a dyslexic child may constitute difficulty while reading

and should be compensated for Improvement in vision will make reading easier for the

dyslexic child

64 The Study Strengths

The study methodology was based on reports found in other studies and consisted of the

following

a Classification The subjects selected for the dyslexic group represent a dyslexic population

that was classified according to the criteria used in the study by Latvala et al 3 and Evans et

139

al 100 The study was undertaken at a school for learning disabled learners and all students

were assessed and categorized as being dyslexic by the psychologists

b Study Examiner Examination of the subjects was conducted by the same optometrist who

had some experience in paediatric optometry This minimised the possibility of bias and

inter-examiner variability

c Examination Techniques The examination techniques used in this study were the same

as the techniques used in major referenced studies which were published in peer-reviewed

journals

d International Relevance It is the first study that has assessed broad vision functions in

a dyslexic population in an African setting

6 5 Limitations of the Study

The factors which may limit the generalization of findings of the study are as follows

a Inattentiveness This occurred in some children after numerous tests and the possibilities

of their lack of understanding of certain instructions However this problem was

minimized as trial readings were taken for some procedures and the participants were

instructed accordingly As stated in Section 34 testing was discontinued when the child

was tired This approach helped to minimize fatigue which could have negatively

affected the results In addition objective techniques that required minimal responses

from the participants were utilized in several procedures such as the cover test

retinoscopy and the monocular estimation method for evaluating accommodation posture

b Convenience Sampling This method of sampling was used given the limited number of

available subjects from the target population The available studies 235687-91102103 on

the subject of dyslexia learning disabilities and vision also used the convenience

sampling method

140

c Generalised Findings The generalization of the findings of this study is limited by the

fact that the refraction data was collected without the use of cycloplegia At the inception

of the data collection the diagnostic drug usage by optometrists was not approved in the

scope of practice in South Africa

d Sample Size The sample size was small as there was only one school for dyslexic

African children in Durban at the time of this study However review of the literature

also reveals that the sample sizes on studies conducted on dyslexia learning disabilities

and vision were typically small The targeted sample size of one hundred for this study

could not be met due to the limited number of subjects available The average sample size

of eight published studies 356 8790100 102 conducted on dyslexic children was 41

e Suppression Control The assessment of binocular accommodation facility and the

vergence reserves were performed without controlling for suppression

f Assessment of Relative Accommodation the use of a Phoropter would have yielded a

more conclusive result

g Eye Movements these were not assessed

Despite the acknowledged limitations inherent in this study the study provides useful

information and a research perspective on the prevalence of vision defects in a South African

population of dyslexic children which has not been conducted before

66 Recommendations

The following recommendations are made based on the findings of this study

661 Future Research

141

1 The same study with a similar protocol larger sample size with randomized sampling

2 Investigate the relative accommodation in in both groups using the phoropter

3 Assess the binocular functions with suppression control

4 Assess refractive error under cycloplgia

662 General Recommendations

1 All dyslexic children should have their eyes examined routinely to rule out

possibility of vision defects impacting on their reading performance

2 The Department of Education be reminded that visual defects can impact negatively on

the reading ability of learners and that teachers need to be more aware of pupils who may

present with signs of visual problems such as holding book too close or battling to see

the chalk board

3 The principals of the mainstream school should also be informed that vision defects were

detected in the non-dyslexic children

67 Conclusion

As a neurological condition which manifests primarily as a language-based disorder with

difficulty with words dyslexia requires a multi-disciplinary approach to its management with

an eye examination being needed to eliminate any possible effects that ocular conditions may

contribute to reading problems

The study provided the prevalence of disorders of visual acuity and refractive errors near point

of convergence accommodation dysfunction heterophoria strabismus and fusional reserves

among African dyslexic children of age range 10-15 years The only vision variable that was

more prevalent and that is significantly associated with dyslexia was the binocular

accommodation facility while only the near point of convergence (break and recovery) was

significantly more prevalent in the control than the dyslexic group The existence of these

statistically significant differences between the two groups may not imply clinical relevance

due to the small sample size The comparison of vision characteristics between the dyslexic

142

and control group indicates that the dyslexic children are not more at risk of having these

vision condition than the non-dyslexic children although sample size was small

Further research on African children will add to the body of knowledge about this group with

respect to the relationship between vision conditions and dyslexia about which very little is

known Only a few studies have provided evidence of the extent of dyslexia among African

children yet even a small percentage could result in many millions of children being affected

given the population of Africa While the problems that result from dyslexia are not life

threatening they do affect peoplersquos opportunities and quality of life

Unfortunately many African countries do not have the resources to provide specialized

schools for children with learning disabilities and every effort therefore needs to be made to

correctly identify the problems and possible causes of dyslexia In order for African countries

to implement the strategies to meet their millennium development education goals of ensuring

that by 2015 all children will be able to complete a full course of primary schooling they

need to provide not only for children in mainstream schools They need also to plan for

children whose academic ability would be enhanced by the appropriate diagnosis of learning

difficulties the provision of a simple eye test and spectacles or corrective apparatus where

appropriate

Dyslexia is not a disease for which a cure can be found rather long-term sustainable

interventions need to be put in place to ensure that the problems are identified and appropriate

remedial action is taken to minimize the impact it has on their life While many African

countries may not be able to afford schools for children with learning disabilities greater

awareness of the manifestation of the condition needs to be made to all teachers in an effort to

provide these children with the possibility of staying in main stream schools for as long as

possible where there are no alternatives With this in mind it is anticipated that this study will

contribute to the body of knowledge on vision conditions of African dyslexic school children

143

REFERENCES

1 Scheiman M Rouse M 1994 Optometric Management of Learning -Related Vision Problems Missouri Mosby Ist Edition

2 Ygge J Lennerstarnd G Axelsson I Rydberg A Visual Function in a Swedish Population of Dyslexic and Normally Reading Children Acta Ophtahlmol(Copenh)1993 71 (1) 1-9

3 Latvala ML Korhoenen TT Penttinen M Ophthalmic Findings in Dyslexic School Children Br J Ophthal 1994 78 339-343

4 Aasved H Ophthalmological Status of School Children with Dyslexia Eye 1987 161-68

5 Evans BJ Drasdo N Richards IL Investigation of some Sensory and Refractive Visual Factors in Dyslexia Vision Research 1994 34 (14) 1913-1926

6 Buzzelli AR Stereopsis Accommodative and Vergence Facility Do they relate to Dyslexia Optom Vis Sc 1991 68 (11) 842-846

7 Naidoo KS Africa Vision Research Institute Monograph 2007

8 Wu SY Nemesure B Leske MC Refractive Errors in a African Adult Population The Barbados Eye Study Invest Ophthal amp Vis Sci 1999 40 (10) 2179-2184

9 Tielsch JM Sommer A Katz J The Prevalance of Refractive errors Among Adults in the United States Western Europe and Australia Arch Ophthalmology 2004 122 495-506

10 Rudnicka AR Shahrul M Owen C Variations in Primary Open-Angle Glaucoma Prevalence by Age Gender and Race A Bayesian Meta-Analysis Invest Ophth Vis Sc 2006 47 (10) 4254 - 4265

11 Raju D 1997 Poor Oculomotor Control in Dyslexia Masters Degree Thesis University of Durban-Westville

12 Keily PM Crewther SG Crewther DP Is there an Association between Functional Vision and Learning Clin amp Exp Optom 2001 84 (6) 346-353

13 Krupska M Klein C 1995 Demystifying Dyslexia London Language and Literacy Unit

14 Schumacher J Hoffmann P Schma C Schulte G Genetics of Dyslexia The Evolving Landscape J Med Genet 2007 44 289ndash297

144

15 Pushpa S Nallur B Biological basis of Dyslexia A Maturing Perspective Current Science 2006 90 (2) 168-175

16 Sherman G International Dyslexia Association Fact Sheet 969 January 2000

17 Berninger V Dyslexia The Invisible Treatable Disorder Learning Disability Quarterly2000 23 (3) 125

18 Harrie RP Weller C Dyslexia httpwwwericecorg Last accessed January 2007

19 Critchley M Is Developmental Dyslexia the Expression of a Minor Cerebral Damage The William Copeland Memorial Lecture National Hospital London 1966

20 Christenson GN Griffin JR Wesson MD Optometrys Role in Learning Disabilities Resolving the Controversy J of Am Optom Assoc 1990 61 (5) 363-372

21 Evans B 2007 Dyslexia and Vision 2nd Edition Whirr Publishers London

22 Flax N Visual function in Dyslexia Am J Optom Arch Acad Optom 1968 45 574-87

23 Optometric Clinical Practice Guidelines Care of the Patient with Learning-Related Vision Problems America Optometric Association 2000

24 Tan L Spinks JA Eden GF Perfetti CA Siok WT Reading depends on Writing in Chinese Pro Natl Acad Sc 2005 102 (24) 8781 ndash 8785

25 httpfacultywashingtonedu What Is Dyslexia Last Accessed 15th September 2008

26 Shaywitz S Overcoming Dyslexia A new and complete science-based program for reading problems at any level 2003 New York NY Alfred A Knopf

27 Holland KC A look at the Reading Process Optician July 3rd 1987 11-13

28 Bedwell CH The Visual and Ocular aspects of Reading Difficulty Optician Sept 24th 1982 26-38

29 Rayner Keith Eye Movement in Reading Perceptual and Cognitive ProcessesCanadian Psychology Feb 1995

30 Sibylina M Brochure from Learning Disabilities Council of America 1998

31 Pavlidis GT Eye Movements The Diagnostic Key to Dyslexia S Afr OptomJuneAugust 1988 29-39

32 Evans B The Role of the Optometrist in Dyslexia Part 1 Specific Learning Difficulties Optometry Today 2004 January 30 29-33

iv

Abstract

Dyslexia is a neurological disorder with genetic origin that affects a personrsquos word

processing ability their spelling writing comprehension and reading and results in poor

academic performance As a result optometrists are consulted for assistance with the

diagnosis and treatment of a possible vision condition Optometrists are able to assist with

treatment as part of a multidisciplinary management approach where optometric support is

necessary International studies have indicated that up to 20 of Caucasian school children

are affected by dyslexia while there are no similar figures for African children Studies have

been done to assess the extent of visual defects among Caucasian dyslexics but not among

African dyslexic children The aim of the study is therefore to determine the prevalence of

vision conditions in an African South African population of dyslexic school children and to

investigate the relationship between dyslexia and vision

The possible relationship between dyslexia and vision conditions has been recognized as an

important area of study resulting in research being conducted in many countries Studies

have been undertaken by optometrists and ophthalmologists who differ in their approach

and attitude on how vision conditions affect dyslexia A review of the literature revealed

three broad areas of vision that may impact on reading ability these being acuity defects

binocular vision and ocular pathology Acuity defects consist of visual acuity and refractive

error Areas of binocular vision evaluated in the literature include near point convergence

heterophoria strabismus accommodative functions vergence facility and reserves

Hyperopia was the only vision variable that was found to be consistently associated with

difficulties with reading but not causally while findings on other variables were

inconclusive However all the studies acknowledged the complexity of the condition and

the need for a comprehensive multidisciplinary management approach for its diagnosis and

management

The study was undertaken in the city of Durban South Africa using a case-control study of

two groups of African school children between the ages of 10 and 15 Both study groups

consisted of 31 children of normal intelligence who were matched in gender race and

v

socio-economic status The case group attended a school for children with learning

disabilities while the control group attended a mainstream school At the time of the study

only one school catered for African children with learning disabilities and only 31 of its

pupils were diagnosed with dyslexia Ethical approval was obtained from the University of

KwaZulu-Natal permission to undertake the study in the identified schools was obtained

from the Department of Education and the school principals consented on behalf of the

learners as it was not always possible to reach the individual parent

The researcher (an optometrist) visited both schools by appointment where rooms were

made available to do the testing and the tests were explained to all participants The

LogMar Acuity Charts were used to assess visual acuity and static retinoscopy was used to

assess refractive error Binocular vision was tested using the cover test for ocular alignment

the Hirschberg test for strabismus RAF rule for near point of convergence 2 D flipper

lenses for accommodation facilities Donderrsquos push up methods using the RAF rule for

amplitude of accommodation plus and minus lenses for relative accommodation monocular

estimation technique for accommodation posture and prism bars for vergence reserves

Ocular pathology was assessed using a direct ophthalmoscope

The dyslexic group presented with the following Refractive errors hyperopia 65

myopia 65 astigmatism 10 anisometropia 65 remote near point of convergence

33 esophoria at near 3 exophoria at near 95 accommodative infacility 54 and lag

of accommodation 3928 The dyslexic group had relatively reduced fusional reserve

compared to the control group

The control group presented with the following Refractive errors hyperopia 3

astigmatism 13 anisometropia 65 remote near point of convergence 48 esophoria at

near 0 exophoria at near 0 accommodative infacility 33 and lag of accommodation

4193

The prevalence of a remote NPC was higher in the control group than in the dyslexic group

and there was a statistically significant difference between the two groups NPC break

vi

(p=0049) and recovery (p=0046) The prevalence of poor binocular accommodation

facility at near was higher in the dyslexic group than in the control group and there was a

statistically significant difference between the two groups (p = 0027)

Vision defects such as hyperopia astigmatism accommodation lag convergence

insufficiency poor near point of convergence and accommodative infacility were present in

the dyslexic pupils but they were no more at risk of any particular vision condition than the

control group This study provided the prevalence of vision conditions in a population of

African dyslexic children in South Africa the only vision variable that was significantly

more prevalent in the dyslexic population being the binocular accommodation facility at

near although the study was unable to find a relationship between dyslexia and vision The

statistically significant difference may not imply clinical significance due to the small

sample size However it is recommended that any vision defects detected should be

appropriately compensated for as defective vision can make reading more difficult for the

dyslexic child

The sample size may have been a limitation however this was comparable with studies

reviewed most of which had sample sizes of less than 41 Due to the range of possible

ocular conditions that could affect dyslexia it is recommended that a larger sample size be

used to ensure more conclusive results Testing for relative accommodation with a

phoropter would provide more accurate results and accommodation facility and fusional

reserves would be better assessed with suppression control The study provides information

and an indication of research needs regarding the prevalence of vision defects in an African

South African population of dyslexic children

vii

TABLE OF CONTENTS

CONTENTS PAGETITLE PAGE iDECLARATION iiACKNOWLEDGEMENTS iiiABSTRACT ivTABLE OF CONTENTS viiLIST OF APPENDICES xiLIST OF TABLES xiiLIST OF FIGURES xiiiLIST OF PUBLICATIONS FROM THIS THESIS xiv

CHAPTER ONE INTRODUCTION 1

11 Background and Rationale for the Study 112 Aim 213 Objective 214 Research Questions 215 Significance of study 316 Goals of Study 317 Null Hypothesis 318 Research (alternative) Hypothesis 419 Scope of Study 4110 Assumptions 5111 Study Outline 5112 An overview of Dyslexia 5113 The Reading Process 8114 Eye Movement in reading 9115 Characteristics Features of Children with Dyslexia 10116 Implications of Dyslexia for learning 11

1161 Reading and Comprehension 121162 Handwriting 121163 Proof-reading 12

117 Prevalence of Dyslexia 12118 Aetiology of Dyslexia 13

1181 Neurological Factors 141182 Genetic Factors 151183 Peri-natal Factors 151184 Substance Use during Pregnancy 151185 Environmental Factors 161186 Nutrition 16

119 Diagnosis of Dyslexia 171191 Exclusionary Method 17

viii

1192 Indirect Method 171193 Direct Method 18

120 Functional Imaging Techniques used in the Diagnosis and Study of Dyslexia 191201 Positron Emission Tomgraphy (PET) 191202 Functional Magnet Resonance Imaging (fMRI) 19

121 Classification of Dyslexia 201211 Primary Dyslexia 211212 Secondary Dyslexia 21

122 Sub-types of Dyslexia 211221 Dysphonetic (auditoryphonetic) 211222 Dyseidetic (visual-spatial) 211223 Dysnemkinesia (motor) 211224 Dysphonetic-Dyseidetic 23

123 Theories of Dyslexia 231231 Phonological-Deficit Theory 231232 Cerebellar Theory 251233 Visual Deficit Theory 261234 Auditory Processing Theory 27

124 Cultural Differences in Dyslexia 28125 Treatment Intervention 30126 Role of Optometry in the Multidisciplinary Management of Dyslexia 30

1261 Case History 311262 Refractive and Binocluar Functions 311263 Visual-Motor Skills 311264 Visual-Spatial Analysis 321265 Short-term Visual Memory (visual-motor recall) 321266 Auditory Analysis Skills 32

127 Optometric Treatment Options 331271 Correction of Vision Defects 331272 The use of Coloured Overlay 33

128 Potential Mechanism for Benefit from Tinted Lenses 35129 Summary of Chapter One 37

CHAPTER TWO LITERATURE REVIEW 39

21 Introduction 3922 Visual Acuity 4023 Refractive Errors 47

231 Myopia 47232 Hyperopia 49233 Astigmatism 49234 Amblyopia 50235 Anisometropia 50

24 Near point of Convergence 53

ix

25 Heterophoria 5526 Strabismus 5827 Accommodation Functions 59

271 Amplitude of Accommodation 59272 Accommodation Facility 61273 Relative Accommodation 64274 Accommodation Posture 64

28 Vergence Facility 6529 Fusional Reserves 66210 Ocular Pathology 68211 Summary of Chapter Two 73

CHAPTER THREE METHODOLOGY 75

31 Introduction 7532 Research Design 75

321 Sampling Design 75322 Study population and Participants 75323 Inclusion Criteria for Dyslexic Group 76324 Exclusion Criteria for Dyslexic Group 76

33 Ethical Clearance 7734 Testing Protocol 7735 Study Setting 7736 Data Collection Technique 78

361 Visual Acuity 79362 Refractive Error 79363 Near Point of Convergence 80364 Strabismus 80365 Heterophoria 81366 Evaluation of Accommodation Functions 83367 Fusional Reserves 85368 Ocular Health 87369 Vergence Facility 87

37 Statistical Analysis 8738 Summary of Chapter Three 89

CHAPTER FOUR RESULTS 91

41 Introduction 9142 Prevalence of Vision Defects 93

421 Visual Acuity 91422 Refractive Errors 92423 Near Point of Convergence 94424 Ocular Health 96

x

425 Heterophoria 96426 Accommodation Functions 97427 Fusional Reserves 101

43 Summary of Chapter Four 105

CHAPTER FIVE DISCUSSION 107

51 Introduction 10752 Prevalence of Vision Defects 107

521 Visual Acuity 109522 Refractive Errors 116523 Heterophoria 116524 Near point of Convergence 118525 Accommodation Functions 120526 Fusional Reserves 126527 Ocular Pathology 130

53 Summary of Chapter Five 130

CHAPTER SIX CONCLUSION 132

61 Introduction 13262 Summary of Findings 134

621 Visual Acuity 134622 Refractive Errors 134623 Near Point of Convergence 135624 Heterophoria 135 625 Accommodation Functions 136 626 Fusional Reserves 137627 Ocular Pathology 137

63 Implications of the Study Findings 138 64 The Study Strengths 138 65 Limitations of the Study 139 66 Recommendations 140

661 Future Research 140662 General Recommendations 140

67 Conclusion 141

REFERENCES 143

xi

LIST OF APPENDICES PAGE

APPENDIX A Clarification of Terms 156APPENDIX B Request for Consent from Department of EducationAPPENDIX C Letter of Consent from Department of EducationAPPENDIX D Letter of Consent from Addington Primary schoolAPPENDIX E Visual Acuity Conversion Table

xii

LIST OF TABLES

TABLE TITLE PAGE

11 Characteristic Features of Dyslexia 1012 Features of the Magnocelular and Parvocellular systems 2721 Conclusions from Major Studies on Dyslexia and Vision 7031 Data Collection Tools and Measurement Outcomes 7832 The Diagnostic Criteria for each Test Variable 8741 Descriptive Statistics for Visual Acuity 9242 Descriptive Statistics for Refractive Error 9443 Descriptive Statistics for Near Point of Convergence Break

and Recovery Points 9544 Descriptive Statistics for Heterophoria 9745 Descriptive Statistics for Accommodative Posture 9846 Descriptive Statistics for Binocular Accommodation Facility 9947 Descriptive Statistics for Amplitude of Accommodation 10048 Descriptive Statistics for Relative Accommodation 10149 Descriptive Statistics for Base-in Vergence Reserves at Distance 102410 Descriptive Statistics for Base-in Vergence Reserves for Near 102411 Descriptive Statistics for Base-out Vergence Reserves at Distance 103412 Descriptive Statistics for Base-out Vergences at Near 103413 Descriptive Statistics for Dyslexic and Control Groups

for all Variables 104

xiii

LIST OF FIGURES

FIGURE TITLE PAGE

41 Prevalence of Visual Acuity 9242 Prevalence of Refractive Errors (total) 9343 Prevalence of the Types of Refractive Errors 9444 Prevalence of NPC Break 9645 Prevalence of Phoria at Near 9646 Prevalence of Accommodation Lag 9847 Prevalence of Reduced Binocular Accommodation Facility at Near 9948 Prevalence of Relative Accommodation 101

1

CHAPTER ONE INTRODUCTION

11 Background and Rationale for the Study

Optometry has long been involved in the subject of vision and learning As primary eye care

practitioners optometrists receive referrals from parents teachers psychologists and other

professionals who usually seek the optometristrsquos advice about whether a childrsquos vision problem

may be contributing to or is responsible for poor academic performance

According to Scheiman and Rouse1 learning which is the acquisition of knowledge or skill can

be achieved through study experience or instruction and that between 75 and 90 of what a

child learns is mediated through the visual pathways As vision plays a major role in reading and

the learning process any defects in the visual pathway disrupt the childrsquos learning

ldquoThe three interrelated areas of visual functions are 1

1 Visual pathway integrity including eye health visual acuity and refractive status

2 Visual efficiency including accommodation binocular vision and eye movements

3 Visual information processing including identification and discrimination

spatial awareness and integration with other senses

Several studies2-6 have been conducted to investigate the relationship between vision and dyslexia

on Caucasian populations while very few have been done on African subjects Consequently

most inferences and conclusions on dyslexia are based on research conducted in Caucasian

populations Naidoo7 reported that inferences drawn from studies in Caucasian populations may

not be appropriate for the African population as the lsquoAfrican Eyersquo differs from the Caucasian eye

For example epidemiological studies have reported variations in refractive errors where the

prevalence of myopia is higher among Caucasian persons than in African persons8 9 while

African populations had higher open angle glaucoma prevalence than Caucasian persons10

This study attempts to contribute to the literature by examining the prevalence of a broader

spectrum of vision variables in dyslexic school children in order to investigate a possible

2

relationship between dyslexia and vision in African school children in South Africa A search of

the literature revealed that very little research has been conducted in Africa the only one being

conducted in 1997 by Raju11 Raju11 investigated oculomotor functions in dyslexic school

children in South Africa and reported that 65 of the normal readers exhibited normal

oculomotor control as compared to 11 of the dyslexics Furthermore 54 of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills while 12 of the

controls displayed the same

12 Aim

The aim of the study was to determine the prevalence of vision conditions in a South African

population of African dyslexic children and to study the relationship between vision and dyslexia

13 Objectives

1 To determine the distribution of visual acuity disorders among African dyslexic children

2 To determine the distribution of refractive errors among dyslexic children

3 To determine the distribution of phorias among dyslexic children

4 To determine the distribution of strabismus among dyslexic children

5 To determine the distribution of accommodation disorders among dyslexic children

6 To determine the distribution of vergence disorders among dyslexic children

7 To determine the distribution of ocular pathology among dyslexic population

8 To compare these findings to a similar group of non-dyslexic children

14 Research Questions

1 What is the prevalence of vision conditions among African dyslexic children of age range

10 and 15 years

3

2 What is the relationship between dyslexia and vision

15 Significance of the Study

The study will attempt to

1 Identify high-risk ocular conditions among dyslexic children which might influence the

need for certain examinations or that could affect the childs ability to learn to read

2 Relate findings of this research work to that of previous researchers

3 Add to the knowledge base in the area of vision and dyslexia

16 Goals of Study

1 To obtain quantitative information on the prevalence of visual conditions in dyslexic

children in the African population

2 To obtain information on the relationship between vision disorders and dyslexia

3 To enable improvement of eye care for children with dyslexia

17 Null Hypothesis

1 There will be no statistically significant difference between the mean visual acuity of the

dyslexic group and of the control group

2 There will be no statistically significant difference between the mean refractive errors of the

dyslexic group and of the control group

3 There will be no statistically significant difference between the mean phoria in the dyslexic

group and of the control group

4 There will be no statistically significant difference between the strabismic mean angle of

deviation of the dyslexic group and of the control group

5 There will be no statistically significant difference between the mean accommodation

functions (amplitude facility posture) in the dyslexic group and of the control group

4

6 There will be no statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

1 8 Research (alternative) Hypothesis

1 There will be a statistically significant difference between the mean refractive errors in the

dyslexic group and of the control group

2 There will be a statistically significant relationship between the mean visual acuity of the

dyslexic group and of the control group

3 There will be a statistically significant difference between the mean of phoria of the

dyslexic group and of the control group

4 There will be a statistically significant difference between the strabismus mean angle of

deviation of the dyslexic group and of the control group

5 There will be a statistically significant difference between the mean accommodation

functions of the dyslexic group and of the control group

6 There will be a statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

19 Scope of Study

1 This study is limited to the assessment of visual functions in dyslexic and normal readers

The visual functions investigated include visual acuity refractive error ocular alignment

near point of convergence accommodation functions and vergence reserves The

assessment of eye movement was not part of the study

2 Screening for and diagnosing dyslexia was not part of this study The school

psychologistsrsquo reports were relied on in selecting subjects who qualify to participate in this

study The information was obtained from the learnersrsquo school files 3 The scope of vision parameters investigated is limited only to the prevalence and

relationship between these variables in dyslexic and control groups as outlined in the

hypothesis Correlation analysis of different variables was not part of this study

5

110 Assumptions

In this report dyslexia is used synonymously with ldquodevelopmental dyslexiardquo ldquoreading

disabilityrdquo and reading disorderrdquo and will be used to describe children who find it extremely

difficult to learn to read despite having normal intelligence appropriate educational opportunities

and absence of emotional disorders Children so defined have reading age that is two or more

years behind their chronological age12

111 Study Outline

This thesis is divided into six chapters Chapter one indicates the rationale of the study provides

an overview of dyslexia and details its purpose objectives significance and goals Chapter Two

(the literature review) gives a detailed review of literature in the area of dyslexia learning

disabilities and vision with particular emphasis to vision variables as outlined in the hypothesis

to be tested This is followed by methodology in Chapter Three which details the research

methodology in which a case-control study was done in two schools in Durban The areas of

vision variables evaluated include visual acuity and refraction binocular vision and ocular health

evaluation Chapter Four (Results) presents an analysis of the results obtained from the study

Chapter Five (Discussion) discusses the study findings and their meaning Chapter Six presents

conclusions from the study based on the study results indicating the implications applications

and recommendations for future study and the study limitations

112 An overview of dyslexia

Some children find it difficult to learn to read despite having normal intelligence appropriate

educational opportunities and absence of emotional disorders Their reading age is two or more

years behind their chronological age Such children are referred to as being dyslexics Where

there is an associated deficit in intelligence the condition may be described as generalized

learning disability12

6

The term dyslexia is derived from the Greek words dys meaning hard or difficult and lexia

from the word lexicos which means pertaining to words so dyslexia means difficulty with

words-either seen heard spoken or felt as in writing13

The World Federation of Neurologists13 and the International Classification of Diseases-1014

defined dyslexia as ldquoa disorder manifested by difficulty in learning to read despite conventional

instruction adequate intelligence and socio-cultural opportunity It is dependent upon

fundamental cognitive disabilities which are frequently constitutional in originrdquo

Dyslexia is a complex disability which affects different aspects of reading The acquisition of

reading related skills are coordinated by visual motor cognitive and language areas of the brain

Consequently dyslexia can result from deviation of normal anatomy and function of cognitive

and language areas in the brain 15

Dyslexia refers to a cluster of symptoms resulting in people having difficulties in specific

language skills particularly reading15 The mystery in dyslexia as a syndrome lies in the fact that

dyslexics have ldquodifficulty to learn to read despite having average or above average intelligence

adequate auditory and visual acuity absence of frank brain damage no primary emotional

problems and have adequate educational instructionsrdquo 16

Dyslexia is a developmental disorder that shows in different ways at different developmental

stages As documented by Krupska and Klein 13 dyslexia is a learning disability characterised by

problems in expressive or receptive oral or written language that manifest initially as difficulty in

learning letters and letter sound association which is followed by difficulty in learning letters and

reading words accurately which consequently results in impaired reading rate and written

expression skills (handwriting spelling and compositional fluency) Some dyslexics find it

difficult to express themselves clearly or to fully understand what others mean when they speak

The effects of dyslexia reach well beyond the classroom as it can affect the persons self-image

It makes them feel dumb13 17 Dyslexia describes a different kind of mind often gifted and

productive that learns differently An unexpected gap exists between learning aptitude and

achievement in school 18

Many dyslexics are creative and have unusual talent in areas such as art athletics architecture

7

graphics electronics mechanics drama music or engineering 18 Some dyslexic learn best with

a multi-sensory delivery of language content which employ all pathways of learning at the same

time seeing hearing touching writing and speaking18 It is primarily not a problem of

defective vision although underlying vision deficit has been known to exaggerate the dyslexics

reading problems 18

According to Critchley19 ldquothe child otherwise normal as regards to intellectual status and

emotional stability finds an isolated difficulty in mastering the significance which lies behind

printed or written symbols and of necessity finds it extremely hard to align the verbal

components of speech (as emitted by the mouth and as received by the ear) with the empirical

characters which are set out on paper in one form or anotherrdquo Dyslexics are unable to acquire

written language skills through ordinary learning and teaching methods and often fail to progress

in education19

A dyslexic individual displays a characteristic pattern of decoding (recognising) and encoding

(spelling) difficulties with written words20 Interestingly dyslexics may not have overt

difficulties with spoken language yet do have marked difficulties with written languages21 The

difference between written and spoken language is that written language is seen while spoken

language is heard This fact may explain why vision has always been implicated on the topic of

dyslexia 21

Dyslexics experience difficulties with visual-verbal matching necessary to establish word

recognition ability Even when a clear single visual image is present the dyslexic may be unable

to decode the printed word in some cases This may be related to the fact that the difficulty that

dyslexics have is at a much more fundamental stage in the reading process so the role of vision

function is quite different in dyslexia than in other types of reading difficulty22 (which may be

peripheral such as hyperopia leading to difficulty in reading) The two main situations in which

the term dyslexia now commonly applies are when the reader has difficulty decoding words (that

is single word identification) and the second is encoding words (spelling) The second type is a

frequent presentation in optometric practice that is when the reader makes a significant number of

letter reversals errors (example b-d) letter transposition in word when reading or writing

(example sign for sing) or has left-right confusion 23

8

113 The Reading Process

Language development entails four fundamental and interactive abilities listening speaking

reading and writing while learning to read involves a complex system of skills relevant to visual

(the appearance of a word) orthographic (visual word form) phonological and semantic

(meaning of word and phrases) processing and a variety of behaviors such as letter naming letter

perception word recognition and comprehension each of which uses a different part of the brain

Reading is an interpretation of graphic symbols24-26

For some children the ability to break a word into its smaller parts a task crucial in reading is

extremely difficult Dyslexia is a localized problem one involving the sounds and not the

meaning of spoken language Speaking is natural while reading is different it must be learned

To be able to transform the printed letters on the page to words that have meaning the letters

must be connected to something that already has inherent meaning- the sounds of spoken

language In order to learn to read children must learn how to link the printed letters on the page

to the sounds of spoken language and understand that words are made up of sounds and that

letters represent these sounds or phonemes24- 26 In other words learning to read requires that the

child be able to break the word into the individual components of language represented by letters

and to be able to tell the difference between the individual phonemes that make up a word

Learning to read requires that the central principle behind the alphabet is understood that is

words are made of phonemes and in print phonemes are represented by letters Phonemes are

the shortest units of sound that can be uttered in a given language and that can be recognized as

being distinct from other sounds in the language For example the word cat has three phonemes

ca ah and ta Blended together the sounds form the word rsquocatrsquo24-26 When one speaks the

sounds are blended ttogether and are said one cat 24-26 To learn to read a child must learn that

there are three separate sounds This is difficult for children with dyslexia The inability to break

word into its parts is the main reason why children with dyslexia have trouble learning to

read24- 26

Furthermore dyslexic children find it difficult to bring the print to language (such as when asked

to read what they just wrote) For example a child can copy the letters w-a-srdquo correctly but

9

when asked what was written a child with dyslexia may reply saw The problem in this case

may not be related to vision but rather one of perceptual skills of what the child does with a word

on a page The brain mechanism of going from print to language is phonologically based 24-26

Reading acquisition builds on the childrsquos spoken language which is already well developed

before the start of formal schooling Once a written word is decoded phonologically its meaning

will become accessible via the existing phonology-to-semantics link in the oral language system

Thus the childrsquos awareness of the phonological structure of sound of speech plays a major role in

the development of reading ability 24-25

114 Eye Movement in Reading

For a person to be able to read a letter or word the eye has to first identify the written material

Normally the eyes scan across the line of words on the page stopping to fixate at various points

then making short jumps from one position to the next (some saccade eye movements are

involved) Typically there are around five fixation pauses in a line Sometimes the eye will

regress and go back to a previously passed over word element A reader views the text mainly

with the controlling or reference eye whilst the other eye diverges and converges within fine

elements27 A sophisticated mechanism27 (a connection of the visual receptors of the retina with

the visual nervous pathway to the brain) 28 ensures control of this drift and prevents diplopia from

being consciously observed These drifts and subsequent recovery can be clearly seen by a

number of techniques including infrared observation of the eyes27

The minimum time a person can spend on fixation and move to the next is around 250

milliseconds27 For skilled readers the average saccade length is 7-9 letter spaces and the

duration of saccades is of the order of 25-50 milliseconds29 while the average fixation pauses last

between 200-250 milliseconds27 29 Thus our eyes are still for some 90 of the time while

reading and are subject to large changes in acceleration and deceleration Even a good reader

will regress and go back 10 to 20 of the time27 29

The textual image seen by the eyes is imaged onto both maculae and transmitted to both cerebral

10

cortex where it is first received in the primary visual cortex (Brodmanrsquos area 17) with some

initial processing of colour and tone occur Progressively higher levels of processing occur in

adjacent areas 18 and 19 with the highest level of visual language processing taking place in the

angular gyrus (area 39) The angular gyrus is central to reading since it is here that hearing

speech and vision meets and the written word is perceived in its auditory form The auditory

form of the word is then processed for comprehension in Wernickersquos area as if the word had been

heard25 27

115 Characteristic SignsSymptoms of Children with Dyslexia

Dyslexia is heterogeneous in nature and there is no single pattern of difficulty that affects all

people Dyslexic children frequently show a combination of characteristics (Table 11)

Table 11 Characteristic SignsSymptoms of Dyslexia 16 30-31

Reading Visionminus Holds book too close

minus Word reversals

minus Skips complete words

minus Re-reads lines

minus Points to words

minus Word substitution

minus Sees double

minus Poor comprehension in oral reading

minus Might see text appearing to jump around on a page

minus Unable to tell difference between letters that look similar in shape such as o and e

minus Unable to tell difference between letters with similar shape but different

orientation such as b and p and q

minus Letters might look jumped up and out of order

minus Letters of some words might appear backwards eg bird looking like drib

11

Spelling

minus Omission of beginning or ending letters

minus Can spell better orally than written

minus Letter reversals

minus Wrong number of letters in words

Difficulty with Reading

minus Difficulty learning to read

minus Difficulty identifying or generating rhyming words or counting syllables in

words (Phonological awareness)

- Difficulty with manipulating sounds in words

minus Difficulty distinguishing different sounds in words (Auditory discrimination)

AuditoryVerbalminus Faulty pronunciation

minus Complains of ear problems

minus Unnatural pitch of voice

minus Difficulty acquiring vocabulary

minus Difficulty following directions

minus Confusion with beforeafter rightleft

minus Difficulty with word retrieval

minus Difficulty understanding concepts and relationships of words and sentences

116 Implications of Dyslexia for Learning

Dyslexia affects people in different ways and depends on the severity of the condition as well as

the effectiveness of instruction or remediation The core difficulty is with word recognition and

reading fluency spelling and writing Some dyslexics manage to learn early reading and spelling

tasks especially with excellent instruction but later experience their most debilitating problems

when more complex language skills are required such as grammar understanding textbook

material and writing essays The dyslexic syndrome therefore affects reading and learning in the

12

following ways263034

1161 Reading and Comprehension

One major difficulty dyslexics have is the time it takes to read and understand a text This makes

reading an extremely difficult task and they may report that they get tired and may get headaches

after reading for a short while30 There is also difficulty in assimilating what they have read even

when they know the words Multiple-choice questions are usually difficult for them as they find

it difficult to recognise words out of context263034

1162 Handwriting

Dyslexics have problems with handwriting since for some forming letters takes concentration

that their ideas get lost and their written expression suffers30

1163 Proof-reading

Dyslexics with visual processing difficulties usually have difficulty in identifying errors in

their own writings Reading difficulties make it uneasy for them to see their own errors in

expression and sentence structure grammar26 30

117 Prevalence of Dyslexia

Estimates of the prevalence of dyslexia in Caucasian populations vary from 5 to 20 in the

general population16 Pavlidis 31 reported that about 1 to 3 of the total population of children

suffers from dyslexia and that 20 to 30 of those cases classified as ldquogeneral reading failures

are probably dyslexics Evans32 reported a prevalence of 4 - 5 in children and stated probably

more boys than girls are dyslexic Park33 documented that up to 15 of the school-age

population has some degree of dyslexia with a ratio of four boys to one girl The large variability

in the prevalence reported by different authorities may be attributed to the differences in the

diagnostic criteria and the cut-off point applied to the psychometric tests14 Dyslexia is the most

common and the most researched type of learning disability34

13

A literature search revealed one study in Egypt that reported the prevalence of specific reading

difficulty in 2nd and 3rd grades in elementary school population to be 1 and the male to female

ratio to be 27 to 1 The authors concluded that the prevalence was low compared to that reported

in western countries and that the difference may be due to the way the Arab language is written

and read 35

Contrary to the above reports that males tend to be more dyslexic than females Guerin Griffin

Gottfried and Christenson 36 studied dyslexia subtypes and severity levels with respect to gender

differences The authors36 concluded that

when objective criteria such as the discrepancy between achievement (in

reading andor spelling) and intelligence or direct assessment of coding

skills are used as criterion and non-referred or random samples are

assessed the ratio of males to females identified as dyslexics approaches

11 When identification depends on referrals from parents and or

teachers more males than females may be detected because of the nature

and intensity of boysrsquo behavioural reactions to the disability

According to Shaywitz and Shaywitz34 good evidence based on sample surveys of randomly

selected population of children now indicates that dyslexia affects boys and girls comparably

118 Aetiology of dyslexia

The literature contains diverse theories (not less than four major theories) on the aetiology of

dyslexia probably due to the complex nature of the subject but several authors13-15 26 37-42 have

maintained that it is a neurological disorder with a genetic origin Despite decades of

multidisciplinary research neuropsychology brain anatomy and neuro-imaging 37-42 the specific

causes of dyslexia are still unknown The leading assumption however is that dyslexia

fundamentally stems from subtle disturbances in the brain and other possible explanations of the

cause of dyslexia such as language processing difficulties are based on the neurological etiology

14

The aetiological considerations in dyslexia are

1181 Neurological Factors

As documented by Habib 37 the concept of the neurological basis of dyslexia was first mentioned

independently by a Scottish ophthalmologist Hinshelwood in 1895 and a British physician

Morgan in 1896 Both found similarities of certain symptoms of dyslexia in some children

Dyslexia was then described as ldquovisual word blindnessrdquo The earlier understandings for the

neurological basis of dyslexia came from neuro-pathological studies of brains from dyslexic

persons According to Habib37 it was first reported by the French neurologist Dejerine in

1891 that damage to the left inferior parieto-occipital region (in adults) resulted in variable

degree of impairments in reading and writing suggesting that the left angular gyrus may play a

role in processing the ldquooptic images of letterrdquo It was subsequently thought that impaired reading

and writing in the young dyslexic patients could be due to abnormal development of the same

parietal region which was damaged in adult alexic patients 37

Another perspective 13 37 is that the dense distributions of ectopias (the presence of neural

elements in places where they are not supposed to be found) all over the cerebral cortex

(particularly in the perisylvian language areas) resulted in alteration of brain organisation One

such alteration in dyslexia is the lack of asymmetry in a language-related cortical region called

the planum temporale (an auditory area that lies on the superior surface of the temporal lobe) In

normal subjects the platinum temporale is usually larger in the left hemisphere However the

dyslexics showed symmetry of the planum temporale 13 37 Furthermore these ectopias may

reduce cortical connectivity as suggested by recent positron emission tomography and magnetic

resonance imaging studies 37

Another speculation was that the development of ectopias could be due to foetal hormonal

(possibly testosterone) imbalances during late pregnancy which could also account for the

possible male predominance in dyslexia 15 30 37 38

The maturation lag which is believed to be due to the slower development of the nerve fibres of

the corpus collosum (bundle of cells connecting the left and right hemisphere of the brain) has

also been theorized to be a cause of dyslexia A delay in the growth of the fibres linking the two

15

hemispheres would result in affected children being physiologically incapable of tasks which

require hemispheric organisation This leads to the inadequate development of the language

functions 12 37 However recent neuro-imaging studies 39-42 have shown that impaired reading of

alphabetic scripts (such as in the English language) is associated with dysfunction of the left

hemisphere posterior reading systems primarily in the left temporo-parietal brain regions

1182 Genetic Factors

Genetic considerations in dyslexia are derived from the fact that dyslexia runs most often in

families However it is speculated that different forms of dyslexia may occur within the same

family which also may means that it seems likely that the inheritance is indirect 15 31 However

genetic markers have been identified in chromosome 18 38 and chromosomes 6 and 15 15 In

fact chromosomes 2 (DYX3) 6 (DYX2) and 15 (DYX1) in dyslexics have been reported to be

inherited in an autosomal dominant mode of transmission 15 Furthermore Ramus 43 stated that

ldquogenetically driven focal cortical abnormalities such as ectopias and microgyri in specific areas

of left perisylvian cortex involved in phonological representations and processing are the

primary cause of dyslexia which is consistent with anatomical studies of dyslexic brains showing

loci of cortical abnormalities functional brain imaging studies showing that the very same areas

are involved in phonological processing and show abnormal activation in dyslexicshelliprdquo

1183 Peri-natal Factors

Peri-natal factors may manifest as problems during pregnancy or complications after delivery

More importantly it can also happen that the mothers immune system reacts to the foetus and

attacks it as if it was an infection It has also been theorized (based on partial evidence) that

dyslexia may occur more often in families who suffered from various autoimmune diseases (such

as rheumatoid arthritis Gravesrsquo diseases multiple sclerosis systemic lupus Sjogrenrsquos

syndrome)3137

1184 Substance Use during Pregnancy

16

Drugs such as cocaine (especially in its smokable form known as crack) seem to affect the

normal development of brain receptors of the foetus It has been documented that mothers who

abuse substance during pregnancy are prone to having smaller babies and such infants are

predisposed to a variety of problems including learning disorders Thirdly alcohol use during

pregnancy may influence the childrsquos development and lead to problems with learning attention

memory or problem-solving There is also a risk of foetal alcohol syndrome (characterised by low

birth weight intellectual impairment hyperactivity and certain physical defects) 30

1185 Environmental Factors

Environmental factors are risk factors for the development of reading disabilities 44 Reading

disabilities which are due to environmental influences have been referred to as non-specific

reading disabilities 20 or pseudo- dyslexia as the reading disorder is secondary to extraneous

factors and genetic factors play an important role 45 ldquoEnvironment factors play a significant role

in the level to which neurological impairments manifest themselves Encouraging nurturing and

stimulating surroundings can reduce the impact of risk factors on learning performance and life

skillsrdquo 44 Environmental circumstances which may profoundly influence the dyslexic child

include parental attitude the role of multilingualism in the home attention problems the

drawback of frequent changes of school the personality of the teacher and the childs emotional

reaction to his difficulties 152030

It seems that the influence of environmental factors may best be described as a potential risk

factor than a causative factor in dyslexia as there are diverse views on the relationship between

environmental influences and dyslexia According to Grigerenko46 socioeconomic level

educational opportunity and home literacy level are major environmental risks for the

manifestations of developmental dyslexia hellip the ldquomagnitude of the effects was not strong enough

to view them as powerful causal factorsrdquo

1186 Nutrition

Nutrition plays an important role in bridging the gap between genetic constitution and a childs

potential for optimal development Nutrition is a basic requirement for the maintenance of

17

growth and overall development while malnutrition and under-nutrition are associated with

disorders in the normal development of intelligence perceptual maturation and academic

achievements 47 Studies conducted by Grant Howard Davies and Chasty 48 showed that there

is an association between dyslexia and low concentration of zinc Zinc is necessary for normal

brain development and function and essential for neurotransmission44

119 Diagnosis of Dyslexia

The first stage in the diagnosis of dyslexia is with awareness by parents or educators that a

problem in reading exists The initial point of assessment starts with the medical practitioner

(possibly a paediatrician) who conducts a complete medical examination and obtains a

comprehensive health history A correct diagnosis of dyslexia starts with a very well taken case

history especially good awareness of the childrsquos developmental history This may give

indications of any medical condition that may be contributing to reading difficulties If indicated

the child may be referred for a neurological examination If dyslexia is suspected the physician

may then refer the child for further evaluation and treatment by a specialist in psycho-educational

assessments In most cases the diagnosis of dyslexia is usually performed by the psychologist

The Psychologists conduct psychometric assessments which include assessment of intelligence

quotient (IQ) various aspects of reading performance spelling mathematics sequential memory

and other aspects of cognitive and emotional development49 50 The major purpose of the

diagnostic process is to isolate the specific difficulties associated with dyslexia and to suggest

appropriate educational intervention 49 50

There are three methods for diagnosing dyslexia

1191 Exclusionary Method

In the exclusionary method the diagnosis of dyslexia is made when all factors involved in a

nonspecific reading disability (reading problems resulting from factors such as intellectual

disability dysfunction of hearing or vision inadequate learning experience socio-cultural

deprivation primary emotional problems poor motivation) are excluded 2051 The problem with

18

the exclusionary method of diagnosing dyslexia is that dyslexia cannot be diagnosed until after

the child has been failing in school for almost two years By this time constant failure may have

produced a negative attitude towards school and psychological problems 31

It provides a very

limited description of the features of the disorder Dyslexia is often defined in terms of the

discrepancy between reading achievement and chronological age or grade level failing to

consider the possible overlap between some of the intelligence quotient tests and reading tests 52

According to Snowling 53 IQ is not strongly related to reading and so the use of IQ in defining

dyslexia is inappropriate stating that many children with low IQ can read perfectly well and that

the discrepancy definition (based on the difference between reading achievement and

chronological age) of dyslexia cannot be used to identify younger children who are too young to

show a discrepancy

1192 Indirect Method

The second method of diagnosing dyslexia is the indirect approach A diagnosis is sought by

attempting to associate neurological soft signs such as finger ldquoagnosiardquo with reading failure The

drawback with this method is that many dyslexics do not manifest neurological soft signs 20

Another type of the indirect method involves the analysis of cognitive tests An example is the

Wechsler Intelligence Scale for Children (Revised) (WISC-R) WISC-R which compares ldquoverbalrdquo

versus ldquoperformancerdquo 20

1193 Direct Method

The third method which seems most reliable is the direct method which involves the use of

characteristic decoding and spelling patterns to determine the specific dyslexia It has the

additional advantage of addressing the fact that dyslexia is heterogeneous 20

19

120 Functional Imaging Techniques used in the Diagnosis and Study of

Dyslexia

There are functional imaging techniques for diagnosing and studying brain activity in dyslexia

These techniques include positron emission tomography (PET) Magnetic resonance imaging

(MRI) and computed tomography (CT) The two major techniques include

1201 Positron Emission Tomography (PET)

PET is a non-invasive imaging technique that uses radioactively labeled compounds to

quantitatively measure metabolic biochemical and functional activity in living tissue It

assesses changes in the function circulation and metabolism of body organs 54 55 The PET is

a sophisticated nuclear medicine technique that looks directly at cerebral (brain) blood flow

and indirectly at brain activity (or metabolism) The most commonly used labels are

flourophores which emit light when stimulated with light of the appropriate wavelength and

radio nuclides which emit gamma rays or beta particles when they decay56 The dose of

radionuclide injected is minute and does not pose any significant hazard Various isotopes are

used depending upon the brain region and function studied54

Unlike MRI or CT imaging which primarily provide images of organ anatomy PET measures

chemical changes that occur before visible signs of disease are present on CT and MRI images 54 While PET is most commonly used in the fields of neurology oncology and cardiology

applications in other fields are currently being studied 57

1202 Functional Magnetic Resonance Imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a noninvasive usually painless procedure that

uses a powerful magnetic field radio waves and a computer to produce detailed pictures of

organs soft tissues bone and virtually all other internal body structures The images can then be

examined on a computer monitor or printed The fMRI does not use ionizing radiation57 58

Functional magnetic resonance imaging (fMRI) is a relatively new procedure that uses MR

imaging to provide a picture of the brains active rather than its static structure The fMRI

20

machines measure the level of oxygen in the blood through a technique called blood level-

dependent (BOLD) contrast The two different states of hemoglobin-oxygen-rich oxyhemoglobin

and oxygen-poor deoxyhemoglobin differ in their magnetic properties The huge fMRI magnets

are sensitive to changes in the concentration of deoxyhemoglobin As neural activity increases

blood flow to the vasculature of the brain presumably increases altering this concentration The

more active a brain area is the more blood flows to it Thus fMRI can provide a moment-by-

moment movie of brain activity

Studies using fMRI are beginning to shed light on dyslexia Functional magnetic resonance

imaging (fMRI) is similar to MRI as both procedures use a similar scanner The difference is that

the fMRI uses somewhat more sophisticated hardware and software that allow it to capture brain

changes (mainly blood flow) as a person performs a specific cognitive task The functional

magnetic resonance imaging (fMRI) scanner measures where blood flows through the brain

during certain tasks26

Earlier studies of dyslexia were based on histo-pathological studies According to Habib37 the

fMRI has an added advantage that cortical anatomy of the dyslexic brain can be demonstrated

thus answering the criticism addressed to results from pathological findings

120 Classifications of Dyslexia

Dyslexia can be classified into two broad categories according to its presumed aetiology The

following methods of classification have been documented by Helveston59

1 Primary dyslexia (specific developmental dyslexia)

2 Secondary dyslexia

1211 Primary Dyslexia

This is considered to be caused by a specific central nervous system defect This neural defect is

thought to be located in the angular gyrus of the dominant hemisphere It is often hereditary and

21

may affect more boys than girls In primary dyslexia the decoding process involved in language

system is affected and reading becomes extremely difficult for the child However intellectual

tasks that do not involve reading text are unaffected 59

1212 Secondary Dyslexia

There are two types of secondary dyslexia59 Endogenous and Exogenous

a Endogenous Dyslexia this results from pathological changes in the central nervous

system secondary to trauma or disease such as childhood meningitis hydrocephalus

with brain damage and porencephalus59 Other medical causes include prematurity

congenital hydrocephalus encephalitis traumatic brain injury and lead or

methylmercury poisoning50

b Exogenous Dyslexia this results from intellectual deprivation usually limited to lack of

adequate educational opportunity or motivation59 or due to low intelligence socio-

cultural deprivation primary emotional problems sensory impairment (visual or

auditory) poor motivation or attention problems20 Reading disability due to these

mentioned factors have been referred to as non-specific reading disability20

1 22 Sub-types of Dyslexia

When children first start to read they learn to recognise simple words by their shapes (sight

analysis) building their sight vocabulary This is followed by learning how to analyse

complicated words by breaking them down into their sound components (phonetic analysis)

The dynamics of the two processes (Sight-analysis and Phonetic analysis) are important in

explaining how words are decoded based on the neurologic-behavioural model documented by

Christenson et al 20 and Evans 21 Dyslexia can be divided into the following subtypes 20 21

22

1221 Dysphonetic (auditoryphonetic)

The individual has a minimal dysfunction involving the Wernickes area Readers have an

extremely difficult time developing phonic word analysis or decoding skills Their typical

reading errors are wild guesses (lsquolikersquo for lsquolittlersquo) errors picking words with similar shapes

(horse for house ) and semantic substitutions (home for house) Their typical spelling errors

are phonetically inaccurate (reffor rough) This type of reader persists in using whole-word

approach and will guess at unfamiliar words rather than using their word analysis skills While

they do not often continue to reverse letters and words after the age of eight they often make

spelling reversals (For example interver for inventor and wirters for writers) Typically

they often look at the first and last letter and the length of the word and then guess (monkey for

money or stop for step)20 21

1222 Dyseidetic (visual-spatial)

In dyseidetic type of dyslexia dysfunction occurs in the angular gyrus The individual have high

ability in phonic word analysis They often have a difficult time learning the letters of the

alphabet They usually have a very low sight vocabulary poor visual memory poor perceptual

skills and letter and word reversals (dig for big or saw for was) Their reversals often last

past the age of eight and include both reading and writing reversals This type of child is often a

very slow reader who often will try to sound out familiar as well as unfamiliar words They spell

the word as it sounds By first identifying the learning style of these children probably during the

case history the optometrist may know the childs strengths and weaknesses Vision therapy can

then be designed to fit each childs needs Enhancing the childrsquos visual perceptual and eye

tracking skills plus having the schools teach these children according to their strengths enable

them to achieve their full potential in school 20 21

122 3 Dysnemkinesia (motor)

Dysnemkinesia is a less serious form of dyslexia and it involves minimal dysfunction of the area

of the motor cortex involved in letter formation Dysnemkinesics can be distinguished by their

23

frequent letter reversals such as d for b as in doy for boy Additional dyslexic types occur

when combinations of the three basic types arise such as dysphoneidesia dysnemkineidesia

dysnemkinphonsia and dysnemkinphoneidesia20 21

1224 Dysphonetic-Dyseidetic

Dysphonetic- dyseidetic reader has a weakness in both whole-word and phonic analysis Their

reading errors include wild guesses (fish for father) and word reversals (no for on) Their

spelling is usually phonetically inaccurate and often bizarre (rit for faster) 2021

123 Theories of Dyslexia

Assisted by the increasing body of knowledge obtained by neurophysiological and imaging

studies14 several theories have been proposed with the aim of characterizing the fundamental

processes underlying dyslexia14 These theories can be classified under four major frameworks

phonological visual cerebellar and auditory

1231 PhonologicalndashDeficit Theory

According to the phonological theory affected individuals have difficulties in perceiving and

segmenting phenomes leading to difficulties in establishing a connection between phonemes and

graphemes 14 The brain recognizes language in a hierarchical order The upper levels deal with

semantics (the meaning of words) syntax (grammatical structure) and discourse (connected

sentences) The lower levels of hierarchy deal with breaking sounds into separate small units

called phonemes (sound units) Thus before words can be comprehended at higher levels in the

hierarchy it has to be broken down into phonologic constituents that the alphabetic characters

represents15 26 An adequate reading development stems from some considerable level of spoken

language already acquired by a child in the early years of life Consequently failure to develop

the association between letters (grapheme) and sound (phoneme) has been considered to be a

major cause of reading and spelling impairment in most cases of developmental dyslexia60-62

24

Reading requires some skills of phonological processing to convert a written image into the

sounds of spoken language1 According to the phonologic-deficit hypothesis people with

dyslexia have difficulty developing an awareness that words both written and spoken can be

broken down into smaller units of sound and that in fact the letters constituting the printed word

represent the sounds heard in the spoken word2

According to Habib37 neuropsychological studies have provided considerable evidence

indicating that the main mechanism leading to reading difficulties is phonological in nature and

that it has been repeatedly demonstrated that the core deficit responsible for impaired learning to

read is phonological in nature and has to do with oral language rather than visual perception At

the brain level this cognitive deficit would arise from a congenital dysfunction of certain cortical

areas involved in phonology and reading60 At the neurological level it is usually assumed that

the origin of the disorder is a congenital dysfunction of the left-hemisphere perisylvian brain

areas underlying phonological representations or connecting between phonological and

orthographic representations60

Brain imaging studies in dyslexics in response to a phonological task indicate under-activation of

posterior brain regions (Wernickesrsquos area angular gyrus and striate cortex) and relative over-

activation in anterior regions (inferior frontal gyrus) These brain activation patterns provide

evidence of an imperfectly functioning brain system for segmenting words into their phonologic

constituents39 42 Since most dyslexics show deficits in phoneme processing the phonological

deficit is the most significant consistent marker of dyslexia and remains the predominant theory15343739 hence the focus on phonology in the majority of remedial programmes 63

Based on the phonological deficit theory the other symptoms of dyslexia are considered as

simple co-morbid markers that do not have a causal relationship with reading disability62

However proponents of the phonological theory typically contend that these disorders are not

part of the core features of dyslexia62 A major setback with the phonological theory is that it

does not account for symptoms such as impairment in visual perception and problems with motor

coordination that are found in dyslexics 15 and does not account for symptoms of dyslexia that are

unrelated to phonetic decoding difficulties such as problems with short term memory and visual

25

processing issues64

The two basic processes involved in reading are decoding and comprehension In dyslexia a

deficit at the level of the phonologic units impairs the ability to segment the written word into its

underlying phonologic elements Consequently the reader experiences difficulty initially in

decoding the word and then in identifying it The phonologic deficit is independent of other non-

phonologic abilities especially the higher-order cognitive and linguistic functions involved in

comprehension such as general intelligence and reasoning vocabulary and syntax are generally

intact A deficit in phonologic analysis and an ironical intact higher-order cognitive ability offers

an explanation for the paradox of otherwise intelligent people who experience great difficulty in

reading 26

According to the phonological model a deficit in a lower-order linguistic (phonologic) function

prevents access to higher-order processes and to the ability to draw meaning from text The

problem is that the person cannot use his or her higher- order linguistic skills to access the

meaning until the printed word has first been decoded and identified Suppose for example that

a man knows the precise meaning of the spoken word apparition however until he can decode

and identify the printed word on the page he will not be able to use his knowledge of the

meaningof the word and it will appear that he does not know the words meaning 26

1232 Cerebellar Theory

The normal pattern of cerebellar asymmetry is anomalous in dyslexia 13 The cerebellar deficit

theory suggests that the automatisation of cognitive processes and motor control in the

cerebellum are disturbed in individuals with dyslexia14 15 62 Evidence for this theory comes

from poor dyslexic performance in coordination balance and time estimation The cerebellar

theory is related to cerebral dominance (greater proficiency of each cerebral hemisphere in

acquisition and performance of certain specific functions) and lateralization (one-sided location

of brain function)13

Physiologically the brain operates cross-laterally thus the left part of the brain controls the right

side of the body and vice versa The human brain is normally asymmetrical due to the

26

specialized functions in the left and right brain However the dyslexic brain appears to be more

symmetrical than the normal brain The consequence of this is that both sides of the brain may

be competing to handle language and therefore less efficient in handling processing language

The left hemisphere is especially important for the production of language The right hemisphere

also has some language capabilities but has no ability to analyse speech sound Secondly the

cerebellum plays a role in motor control and therefore in speech articulation It is postulated that

retarded or dysfunctional articulation would lead to deficient phonological representations

Furthermore the cerebellum plays a role in the automatization of tasks such as driving typing

and reading A weak capacity to automatize would affect among other things the learning of

grapheme-phoneme correspondences 13-15 62

1233 Visual Deficit Theory

For many years a major view was that reading difficulties are caused by dysfunction in the

magnocellular subsystem 65-67 The visual theory does not exclude the phonological deficit but

emphasizes a visual contribution to reading problems in some dyslexics The biological basis of

the proposed visual theory is derived from the division of the visual system into two sub-systems

which carry information to the visual cortex the transient and the sustained systems62

Anatomically the magnocellular pathway receives input from both rods and cones across the

retina and extends from ganglion cells in the retina to the two ventral layers of the lateral

geniculate nucleus (LGN) The large cell bodies in these two layers give the pathway its name It

projects via the visual cortex and the dorsal or ldquowhererdquo stream to the parietal cortex The

parvocellular (or sustained) system on the other receives input from cones and therefore mainly

the central retina and runs from retinal ganglion cells to the smaller cell bodies of the four dorsal

layers of the LGN It projects via the visual cortex and the ventral or ldquowhatrdquo stream to the

infero-temporal cortex and mediates colour vision and detection of fine spatial details61 The

magnocellular system is the first to respond to a stimulus and relies on further detail from the

parvocellular system (Table 12)

27

Table 12 Features of the Magnocellular and Pavocellular Systems2168

Magnocellular(transient) System Pavocellular (sustained) SystemLarge cells Small cells

Dim light Bright light

High contrast sensitivity Low contrast sensitivity

Low spatial frequency High spatial frequency

Peripheral Central

Right brain Left brain

Midbrain Cortical

Non-cognitive Cognitive

Emotional Logical

Hyperopia Myopia

Gross depth perception Stereopsis

Achromotopsia Colour vision

Fast transmission Low transmission

Sensitive to large stimuli Sensitive to small stimuli

The transient system is vital to the timing of visual stimuli and plays an important part in

detecting the movement of objects (motion perception) and ultimately their signals are used to

control eye and limb movements made in relation to visual targets1521 Impaired function of

magnocellular pathway will lead to destabilization of binocular fixation which results in visual

confusion and letters then appear to move around

Dyslexicsrsquo eyes are unsteady when they are attempting to view small letters hence their vision is

unstable they tend to make visual reading errors15 There is a tendency to superimpose letters on

top of each other and mis-sequence letters in a word1621 Evidence for magnocellular

dysfunction comes from anatomical studies of dyslexic individuals showing abnormalities of the

lateral geniculate nucleus1562 psychophysical and neuroimaging studies 62 64 showing decreased

sensitivities in the magnocellular range (low spatial frequencies and high temporal frequencies)

in dyslexics

28

A deficit in the magnocellular system slows down the visual processing system resulting in an

unstable binocular control inaccurate judgements of visual direction a tendency to superimpose

letters and mis-sequence of letters in a word16 21 Some studies65-67 have shown evidence in

support of the magnocellular deficit theory of dyslexia

1234 Auditory Processing Theory

The auditory processing theory specifies that the deficit lies in the perception of short sounds and

fast transitions (called the ldquorapidrdquo or ldquotemporalrdquo auditory processing deficit) Such a

characterization of the auditory dysfunction is consistent with the magnocellular theory since

magnocellular cells are particularly sensitive to high temporal frequencies60 62

Proponents of the auditory processing theory hypothesized that impaired perception of brief

sounds and transitions would be particularly detrimental to speech perception hence would

undermine the development of the childrsquos phonological representations Contrary to this

hypothesis Rasmus60 reported that recent studies have now adequately confirmed that there is

no reliable relationship between performance on rapid auditory processing tasks and speech

categorization and discrimination neither is there a reliable relationship between any auditory

measure (speech or non-speech) and more general measures of phonological skill or reading

ability even when assessed longitudinally A possibility is that the most auditorily impaired

dyslexics also have severely impaired phonology and reading although the reverse is not

necessarily true 60

Despite the fact that evidence exist in support of the theories and that some of the deficits found

in affected individuals are correlated with reading and spelling they may not be causally

associated with dyslexia However results from genetic research may have the potential to help

delineate which cognitive and neurophysiological processes are causally related14

124 Cultural Differences in Dyslexia

The recognition of dyslexia as a neuro-developmental abnormality has been influenced by the

belief that it is not a specific diagnostic entity because it has variable and culture-specific

29

manifestations 69-71 Recent neuro-imaging studies39-42 have demonstrated that impaired reading

of alphabetic scripts is associated with dysfunction of left temporo-parietal brain regions

However it has been reported by Siok et al in two separate studies69 70 that in contrast to the

assumption that dyslexia in different languages has a universal biological origin their study using

functional magnetic resonance imaging with reading-impaired Chinese children and associated

controls showed that functional disruption of the left middle frontal gyrus is associated with

impaired reading of the Chinese language (a logographic rather than alphabetic writing system)

The neural connections involved in reading and reading disorders may vary in different languages

because of differences in how a writing system links print to spoken language69 70 In Chinese

for example graphic forms (characters) are mapped to syllables which differ markedly from an

alphabetic system (such as English) in which graphic units (letters) are mapped to phonemes

These differences can lead to differences in how reading is supported in the brain69 70 Readers

of Chinese show relatively more engagement of visuo-spatial areas and left middle frontal

regions for verbal working memory presumably for recognizing complex square-shaped

characters whose pronunciations must be memorized instead of being learned by using letter-to-

sound conversion rules69 70

In two separate studies Siok Perfetti and Tan69 and Siok Zhen Charles Perfetti and Tan 70

studied language-related activation of cortical region of Chinese dyslexic school children using

the functional magnetic resonance imaging and found a reduced activation in the same left

middle frontal gyrus region in Chinese dyslexics Chinese dyslexics by contrast did not show

functional or structural differences from normal subjects in the more posterior brain systems that

have been shown to be abnormal in alphabetic-language dyslexics69-70 The authors concluded

that the results of the studies suggested that the structural and functional basis for dyslexia varies

between alphabetic and non-alphabetic languages and that the findings provided an insight into

the fundamental patho-physiology of dyslexia by suggesting that rather than having a universal

origin the biological abnormality of impaired reading is dependent on culture

The study by Paulesu Demonet Fazio and McCrory71 gave another perspective Using the

positron emission tomography to study brain activity of dyslexics from three countries (Italian

French and English) the authors found the same reduced activity in a region of the left

30

hemisphere in dyslexics from all three countries with the maximum peak in the middle temporal

gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus

Their study showed that there was a universal neuro-cognitive basis for dyslexia and that

differences in reading performance among dyslexics of different countries are due to different

orthographies Languages with transparent or shallow orthography (such as Italian) the letters

of the alphabet alone or in combination are in most instances uniquely mapped to each of the

speech sounds occurring in the language Learning to read in such languages is easier than in

languages with deep orthography (such as English and French) where the mapping between

letters speech sounds and whole-word sounds is often highly ambiguous 71

125 TreatmentIntervention

Individualized instruction of dyslexics emphasizing increased phonologic awareness can have a

favorable long-term effect on academic achievement Individualized instruction aimed at

increasing phonologic awareness decoding skills sight word vocabulary and reading

comprehension are particularly helpful to school aged children experiencing difficulties with

reading Most importantly intervention (provided by expert teachers) should be initiated early50

Reading disabilities require lifelong assistance and optimal management strategies differ

depending on the patientrsquos age and circumstances In early childhood the focus is on

remediation of reading often with an emphasis on increasing phonologic awareness Other

strategies include using audio books and modified homework assignments For secondary and

college students intervention focuses on accommodations which include extra time for reading

tape recorders in the classroom audio books instruction in word processing and the use of a

spell-checker (poor phonemic association also causes problems in spelling)3450

126 Role of Optometry in the Multidisciplinary Management of

Dyslexia

The role of the optometrist in the multidisciplinary management of dyslexia starts with an attempt

31

to identify characteristics which may indicate that the child may be dyslexic A complete visual

examination should be conducted Any vision anomalies detected should be corrected and a

prompt referral should be made to the appropriate specialist in the diagnosis of dyslexia and a

follow up should be made by the optometrist

A comprehensive visual evaluation should include1 72

1 Evaluation for a sensory problem or refractive anomaly

2 Evaluation for binocular vision problems

3 Evaluation for ocular pathology

4 An evaluation of visual motor skills (required for drawing and handwriting) eye movement

control skills and visual imagery skills

5 Evaluation of visual information processing including visual spatial skills

(rightleft discrimination) visual analysis skills (matching and discrimination skills)

The role of the optometrist in the management of reading dysfunctions is to identify existing

vision disorders which can be achieved by evaluating the following aspect of vision functions

1261 Case History

A detailed case history is paramount for children who present with reading dysfunction A

thorough investigation of the pre-natal peri-natal and post-natal risk factors is important to

establish the possibility of major developmental delay For example maternal history of

alcoholism may indicate foetal alcohol syndrome Prematurity (early birth date of less than 37

weeks or low birth weight of less than 2500g) is also a factor in later learning A working

knowledge that may not necessarily mean expertise in all related areas may be adequate for the

optometrist72

1262 Refractive and Binocular Functions

A thorough investigation and compensation for refractive and binocular vision functions should

be undertaken4472

1263 Visual-Motor Skills

32

VisualndashMotor integration is the ability to coordinate visual information-processing skills with

motor skills44 It is a very vital skill in academic achievement4472 Children suffering from visual-

motor dysfunction have difficulty copying written work accurately and efficiently Difficulty

with copying can reduce the speed and accuracy of writing which subsequently affects

learning4472 The Beery Test for Visual-motor Integration is particularly useful for the primary

care optometrist It requires only a few minutes to administer with an added advantage that the

scoring guide provides many examples of the pass-fail criterion for each item tested72

1264 Visual-Spatial Analysis

ldquoThe visual-spatial skills allow the child to make judgments about location of objects in visual

space in reference to other objects and to the individuals own body It develops from awareness

within the individualrsquos body of concepts such as left and right up and down and front and back It is important for good motor coordination balance and directional sensesrdquo 72 The Rosnerrsquos

Visual Analysis Skills programme is useful for the assessment of both visual-motor skill and

spatial analysis72

1265 Short- Term Visual Memory (Visual-Motor recall)

The visual memory test assesses the ability to recognize and recall visually presented material 4473 The ability to copy written material accurately is more or less a matching process when the

written materials are close (such as copying from workbook to paper) When the reading

material is separated from the actual visual-motor task such as copying from chalkboard to

paper the process becomes more complex An example of short-term visual memory skill

assessment test is the Getman-Henderson-Marcus Test of Visual recall72

1266 Auditory Analysis Skills

Screening for auditory-perceptual skill at the primary care level is essential as the ability to

analyze spoken language into separate sounds and sound sequence is directly related to reading

and spelling success Examples of tests to measure auditory perceptual skills are the Test of

33

Auditory Analysis Skills (TAAS) (which is easy to administer and score) and a more complex

one Test for Auditory-Perceptual Skills72

127 Optometric Treatment Options

The optometric treatment options for the correction of vision defects include the following

options

1271 Correction of Vision Defects

1 Compensation for refractive error

2 Treatment of visual efficiency deficits with lenses prisms and vision therapy

3 Treament of vision information processing deficits with vision therapy commencing with

visual spatial orientation then visual analysis and concluding with visual-motor

integration

4 Referral to another health care professional educational system and or psychologist

should be considered at any time if underlying physical or neurological problems cognitive

deficits or emotional disorders are suspected1 50 72

The expected outcome of an optometric intervention is an improvement in visual function with

the reduction of vision anomalies associated with reading which makes educational

intervention easy1 Solan47 proposed that the role of optometrists should not end with

correcting vision anomalies in dyslexic patients but that optometrists should participate in

community services to assist with the developmental environment of those whose genetic

make-up may predispose them to having reading difficulties

1272 The Use of Tinted Lenses and Coloured Overlays

The patients rely on professionals to attend to their vision care needs Consequently it is

imperative that we practice at the highest level of professionalism and ensure that anything we

prescribe for our patients are supported with credible research

As reported by Evans74 75 Olive Meares in 1980 suggested that some childrenrsquos perception of

text and certain cases of reading difficulties were influenced by print characteristics which

34

subsequently resulted in visual perceptual anomalies such as words blurring doubling and

jumping Subsequently Meares and Irlen proposed that ldquoScotopic sensitivity syndrome or

(Meares -Irlen syndrome) ndash a syndrome of visual symptoms and distortion can be alleviated

with coloured filters Meares and Irlen claimed that the coloured overlays improve reading

ability and visual perception increase sustained reading time and eliminate symptoms

associated with reading such as light sensitivity eyestrain headaches blurring of print loss of

place and watery eyes75 This type of symptoms is reported both by people who experience

frequent severe headaches of the migraine type as well as some people with dyslexia76

Following this initiative Irlen developed a proprietary system in countries like the United States

of America United Kingdom and Australia to detect and treat Meares-Irlen syndrome Meares-

Irlen syndrome is reported to be treated with coloured filters either coloured sheets (overlays)

placed on the page or colored lenses Computer users may attain a similar effect by changing the

colour of the screen background and or text while people that consistently work under the same

lighting conditions can benefit from the colour therapy by simply varying the colour of the

illuminating light75

The earlier studies of Irlen was criticized for its lack of published double masked studies with a

placebo control to support her claims that the colour that helped had to be specific for each

person76

According to Lightstone76 the Intuitive Colorimeter (developed by Professor Wilkins) of the

Medical Research Council in the United Kingdom was the first to be used to conduct a masked

trial With this instrument it was possible to determine the optimum colour for the spectacle lens

without placing lenses in front of the eyes and could be done in a way that the subject is unaware

of the exact colour they had selected At the end of the study it was felt that the use of colour

was credible and was then included as part of a routine assessment of people with specific

learning difficulties at the Institute of Optometry London76

As described by Lightstone76 the Intuitive colorimeter is basically a large box which the patient

or subject looks into The space that the subject looks at is entirely diffused by a coloured light

and so creates the effect of wearing coloured lenses There are no other colours in the field of

35

view and so no reference colours are available to help the subject decide the exact hue (colour) of

the light that they are viewing This encourages rapid colour adaptation The person operating

the colorimeter changes all aspects of the colour that the subject is viewing and the hue

saturation (depth of colour) and the brightness of the colour can be varied independently of each

other Although the procedure is subjective the technique shows whether there is a specific

colour that gives relief from the symptoms If the effect is placebo the results tend to be variable

and no specific colour can be selected as the optimum colour75 76

An advantage of the Intuitive colorimeter is that because of colour adaptation the person is

unaware of the exact color of the light shining on the text74 However a major demerit with

double-masked trial was that the results were purely subjective and there were no objective

measurement of any improvement with the colour 76

128 Potential Mechanism for Benefit from Tinted Lenses

Some theories have been proposed to be the mechanism of benefit from tinted lenses However

Evans 49 suggested that these hypotheses have not been able to account for the high degree of

specificity of the required colour which has been emphasized by Irlen and substantiated by

double-masked randomized placebo-controlled trials

The theories are

a Pattern Glare The basis of the theory on how pattern glare explains the benefits of tinted

lenses is that certain cells within the visual cortex are hypersensitive and so causes

discomfort while viewing patterns of repeating black and white lines on a page 75 Since

some of the neurons in the visual cortex are sensitive to specific colours varying the colour

of the illuminating light may change the pattern of excitation within the cortex This could

account for the benefit from specific coloured filters From this it has been hypothesized

that by changing the colour of the input the stimulus is transmitted to other cells that are

not hypersensitive and so prevent the distortions and discomfort occurring 7576

b Syntonics Syntonics is a colour vision therapy system used by some optometrist for the

treatment of several conditions including strabismus amblyopia and reading disorders

36

The therapy involves subjects viewing a light source covered by a colored filter The theory

is that the colour influences the endocrine system through the inferior accessory optic tract

The colour seems to be chosen on the basis of a hypothetical connection between an alleged

autonomic nervous system imbalance and the type of heterophoria Treatment is said to

result in improved visual fields7475

c Placebo Effect The placebo effect is one of the possible mechanisms for the benefit from

tinted lenses People may believe that a treatment works simply because it is claimed to be

effective74

Williams and Kitchner77 conducted a literature review on the use of tinted lenses and colored

overlays for the treatment of dyslexia and other related reading and learning disorders and

concluded that

1 there was evidence that the symptoms associated with Irlen syndrome are related to

identifiable vision problems such as binocular and oculomotor problems and that condition

returned to normal when treated with lenses prisms or vision therapy When patients

exhibiting the Irlen syndrome were treated with vision therapy the symptoms were relieved

and therefore did not demonstrate the need for colored lenses

2 most investigators did not control for the presence of vision anomalies

3 the results of prospective controlled research on the effectiveness of tinted lenses or colored

overlays varied

4 the results of testing used to determine the most appropriate color were not repeatable

5 the effects of spectral filters and colored overlays were not solely a placebo Colored

overlays and tinted lenses are not cures for dyslexia but may be useful reading aids for

some individuals with reading difficulty

6 there was a lack of agreement about the best ways to evaluate patients for the presence of

Irlen syndrome

The American Optometric Association78 issued a policy statement on the use of tinted lenses and

colored overlays which states as follows

37

1 Undetected vision problems may be a factor in individuals who exhibit the symptoms of

the Irlen Syndrome A comprehensive eyevision examination with particular emphasis

on accommodation binocular vision and ocular motor function is recommended for all

individuals experiencing reading or learning difficulties as well as those showing signs

and symptoms of visual efficiency problems

2 The American Optometric Association encourages further research to investigate the

effect that specifically tinted lenses and colored overlays have on visual function related

to reading performance

3 Vision problems are a frequent factor in reading difficulties Ignoring the role of vision

or inadequately evaluating the vision of individuals with reading problems is a disservice

which may prevent the person from receiving appropriate care

In concluding Lighthouse76 advices that until more is known about prescribing specific colored

lenses optometrists should be certain to offer the patient the best advice According to Wilkins 79

some 20 of unselected children in mainstream education (not simply those with reading

difficulties) find colored overlays of benefit and the benefit is pronounced in 5 Wilkins79

emphasized that ldquocolored filters do not provide a treatment for dyslexia ndash they provide a means of

avoiding the visual stress with which reading is sometimes associatedrdquo

129 SUMMARY

Dyslexia is a neuro-developomental condition that impacts on a persons spelling

comprehension writing and reading and not only limits their academic career but also impacts

on all areas of life that require these skills Its diagnosis management and treatment require a

multidisciplinary team as it can present with many characteristics which requires careful

classification While it is not always possible to identify the cause it is generally considered to

be a neurological condition with a genetic origin It affects up to one fifth of Caucasians with

limited studies being done on other ethnic groups including Africans

The aim of the study was therefore to investigate the prevalence of vision defects in an African

38

South African population of dyslexic schoolchildren with an attempt to identify high risk

visual conditions that may be prevalent It was hypothesized that there was no relationship

between vision defects and dyslexia

The reading process requires learning to transform printed letters on a page to words that have

meaning Words are made up of phenomes which are represented by letters and it is the

inability to break words into parts that result in the manifestation of dyslexia Its characteristics

include letter reversal faulty pronunciation difficulty with word retrieval and learning to talk

As all visual stimuli is transmitted through the eye there is an association between ocular

conditions and problems associated with letter words and reading hence the involvement of

optometrists The primary role of optometry is to identify and compensate for visual defects

that may constitute an impediment to reading Evaluation by an optometrist is therefore

important to assess and provide advice or devices where appropriate

Initial identification usually takes place by teachers or parents and may result in the child

being sent to a school equipped to deal with learning disabilities Psychologists diagnose

dyslexia using some range of psychometric tests The advent of imaging techniques such as

positron emission tomography and functional magnetic resonance has aided research in

dyslexia as these techniques enable the study of dyslexic brain activity as a reading task is

performed The two broad categories of dyslexia one being due to a genetic neurological

defect and the other being due to a secondary incident that impacts on the brain There are a

number of theories regarding the underlying causes the only theory related (still controversial)

to visual system being the visual processing theory

Chapter One provided the study aims objectives hypotheses to be tested background

information on dyslexia and rationale for the study while Chapter two will review the related

literature on dyslexia and vision

39

CHAPTER TWO LITERATURE REVIEW

21 Introduction

The relationship between vision and dyslexia has been a subject of controversy in optometric

and ophthalmologic literature A review of existing literature 1-32 on the area of vision and

dyslexia reveals that the studies conducted to investigate visual functions in the dyslexic

subjects do not establish a causal relationship between vision and dyslexia but tend to

establish associations between dyslexia and certain vision variables The literature review also

shows that there exists a dichotomy between the ophthalmologists and optometrists on the

subject of dyslexia and vision The difference in perspective can be seen from the style of

report in studies conducted by optometrists 5683-849198100-101 and by ophthalmologists 2-4 85 95

99102 Optometrists maintain that although vision may not be considered as a cause of dyslexia

compensating for visual defects make reading easier for the dyslexic child Ophthalmologists

hold the contrary view that vision does not in any way contribute to dyslexia Dyslexia has

also been thought to be a language-based learning disability80 Consequently from the speech-

language pathologistsrsquo perspective ldquoa child with a speech deficit may also have an emotionally

distorted perspective and a child whose classroom behaviour is hard to control may also be

unable to readrdquo The role of the psychologists is to perform psychometric assessments of the

dyslexic child32 The educator is primarily involved with special educational intervention for

the dyslexic child

Flax 22 proposed that the role of visual factors in dyslexia ldquodepends on the definition of vision

that is consideredPrime meaning that if vision is narrowly defined as visual acuity or clarity of

sight it is unlikely that there would be any relationship between vision and dyslexia If other

peripheral visual functions such as fusion convergence refractive errors and accommodation

are considered there is likely to be a relationship but then questioned the possibility of vision

being a causative factor Flax22 reiterated that the most important aspect of vision that is

closely linked to dyslexia is when vision is defined to include perceptual and integrative

aspects of vision It was concluded that ldquovision difficulties are part of the dyslexic

40

syndrome22

From a similar perspective Solan81 suggested that vision anomalies are contributory and

frequently hinder responses to educational intervention He emphasized that in cases where

vision anomalies are factors involved in the reading disability the effort required to maintain

clear single binocular vision decreases the efficient organization of the visual cognitive

response

This review has been structured in a way that an overview of a study will be made when it is

first cited Subsequently such studies will be cited only with references to the vision variable

being reviewed Only the studies that relate directly to the topic of the present research such as

visual acuity refractive errors near point of convergence ocular alignments accommodation

functions and vergence reserves are reviewed

It is also important to note that there are many variations in the results reported by different

authors In view of this the findings are thus presented on an individual basis Various studies

have attempted to study the relationship between dyslexia and vision and have examined the

different aspects of vision suspected of affecting reading ability However the major

drawbacks that plague the researches in the subject are Firstly intelligence quotient is a factor

to consider when defining dyslexia However only a few studies587-89 consulted have

considered intelligent quotient in their subjectsrsquo selection

Another area worth mentioning is the fact that some of the studies lacked control group which

made the reports difficult to assess and utilize In addition some researchers failed to classify

some visual functions such as binocular coordination and refractive errors thereby making it

difficult to evaluate the impact of a particular vision variable on reading ability Furthermore

some studies failed to indicate the eye examination techniques used in their protocol which

makes the replication of such studies difficultGiven the inherent limitations with studies

conducted to investigate vision in dyslexic children these studies are important in guiding the

teacher and clinicians on the appropriate intervention for the child

22 Visual Acuity

41

Vision plays an important role in the reading process as the ability to see printed material is

crucial in reading Reading efficiency and speed starts to decrease when the print size is less

than three times larger than an individualrsquos visual acuity threshold A marked bilateral

reduction in visual acuity can definitely limit reading performance but it is unknown if

deficient visual acuity characterizes students who have difficulty to read82

Available studies3-58384 that have attempted to see how visual acuity affects reading

performance are presented starting with the first three studies in this review that found a higher

prevalence of reduced visual acuity among dyslexics compared to normal readers

In 1994 Latvala Korhoenen and Penttinen 3 assessed the ophthalmic status of fifty five (55)

dyslexic and fifty (50) control subjects in an age gender and social class matched study in

Finland The subjectsrsquo school grade and intelligence quotient were not indicated Since

intelligence quotient is a major factor in the definition of dyslexia the lack of information on

intelligence quotient may lead to the questioning as to if the population studied were truly

dyslexics The authors assessed visual acuity using the Snellen chart and found that for the

control group all subjects had visual acuity of 07 (69) while in the dyslexic group two

children (36) had bilateral visual acuity of less than or equal to 07

A study to examine the lsquoophthalmologicalrsquo status of school children with dyslexia in Norway

was conducted by Aasved 4 in 1987 Visual examinations were performed on two hundred and

fifty nine dyslexic children The number of the participants from the control group was not

clearly stated The investigation techniques used was also not detailed As a result it may be

impossible for other researchers to replicate the study However Aasved 4 reported that the

dyslexic population had a higher prevalence of reduced visual acuity than the normal readers

did It was concluded that although there was no causal relationship between eye

characteristics and reading difficulties eye abnormalities should be corrected when detected in

dyslexic children

42

Evans et al 5 in 1993 studied some sensory and refractive visual factors in 39 dyslexic and 43

control subjects aged between seven years six months and twelve years three months in the

United Kingdom Participants from both groups had intelligence quotient of above 85 The

authors found that the dyslexic group had a significantly worse near and distance binocular

visual acuity the inter-ocular difference was not statistically different in the two groups and

the difference between the best monocular and the binocular visual acuity was not statistically

different between the two groups at distance or near They claimed that the ldquoreduced visual

acuity may not have been due to refractive defectsPrime Apart from refractive errors a reduced

visual acuity may arise from pathological conditions such as cataract and glaucoma

The following two studies reported similar prevalence of visual acuity worse than 69 In 1980

Hoffman83 investigated the incidence of vision difficulties in learning disabled children in the

United States of America Vision evaluations were performed on one hundred and seven

children with learning problems who were referred to the clinic for vision care from educators

and psychologists The subjectsrsquo mean age was eight years eight months As a comparison

(control) group twenty five patients between the ages of five and fourteen were selected from

the general optometry clinic The prevalence of visual acuity poorer than 69 was reported to

be 1028 among the learning disabled subjects The subjects from the control group were not

appropriately selected not representative enough and were not age-matched although the issue

of the comparison group being inappropriately selected was acknowledged by the author

Another limitation with the study was that no information was given on the eye examination

procedures followed This may be problematic as it makes comparison with other studies and

replication of the study difficult

Sherman84 conducted a study to ldquorelate vision disorders to learning disabilitiesrdquo in the United

States of America in 1973 Thirty-nine boys and eleven girls participated in the study The

children were diagnosed as having learning difficulties by the referring psychologists Their

43

ages ranged from six to thirteen years The technique used to measure visual acuity was not

indicated but it was reported that five children (10) had visual acuity poorer than 69

Other studies reported the prevalence of visual acuity to be similar between the control and

reading disabled group and include the following studies In 1985 Helveston Weber and

Miller85 (from ophthalmology) studied visual functions and academic performance in first

second and third grade children in the United States of America One thousand nine hundred

and ten children participated in this study The Snellen chart and the Jaegar print were used to

assess distance and near visual acuity respectively The authors reported that 94 of the

children tested had normal visual acuity and that visual acuity was not related to academic

performance in a positive way Although the study may be credited with a large sample size a

possible limitation with the application of the findings of this study was that the passfail

criteria were arbitrarily chosen by the examiners ldquoon the basis of previous clinical experiencerdquo

without references to other research studies Secondly the authors failed to present the results

of the statistical analyses of the relationships between vision functions and reading

Grisham and Simon82 conducted a review of literature on refractive error and reading process

and reported that there are abundant and reliable scientific evidence that shows a higher

prevalence of vision defects among reading disabled children than among normal readers It

was emphasized that there are cases where prescription of glasses has improved academic

performance Their analysis revealed no statistically significant difference in distance visual

acuity among normal and disabled readers but that near visual acuity may be an index which

distinguishes some children who are having difficulty to read and those who do not

It is important to note that since reading is a near point task a deficient near visual acuity may

be more contributory to poor reading than reduced distance acuity Consequently Grisham

and Simons82 stated that ldquodeficient distance visual acuity would not be expected to affect the

44

reading skills of children unless substantial reading instructions take place on the chalk board

It is possible that distance acuity is a factor in the early grades when reading is first

introducedrdquo

In 2007 Grisham Powers and Riles86 studied visual skills of 461 poor readers in California

high schools The participants mean age was 154 years and there was no control group in the

study Participants for the study had been identified by the schools as poor readers The vision

functions measured were visual acuity near point of convergence ldquoconvergence and

divergence and fusional rangesrdquo accommodative amplitude and accommodative facility No

information was given on IQ but it was mentioned that the basis for referral of students for

assessment was poor reading performance which was determined by the school and defined as

reading two grade levels or more below grade level Visual acuity was assessed using the

Snellenrsquos chart at six meters The authors reported that 568 of the participants had visual

acuity of 66 in both eyes 261 had visual acuity of 69 in each eye while 172 had visual

acuity of worse than 69 in either eye

Another comprehensive study conducted in 2007 was reported by Kapoula Bucci and

Jurion87 Convergence and divergence functions in 57 dyslexic and 46 non-dyslexic age-

matched children were studied in French dyslexic children The mean age of the dyslexic

subjects was 113 years the mean IQ was 105 and the mean reading age was 891 The mean

age for the control group was 107 years The IQ score of the control subjects was not

indicated The vision functions performed included visual acuity stereo acuity and cover test

near point of convergence and ldquomeasurement of convergence fusional amplitude (divergent or

convergent) Unfortunately the authors did not detail the testing procedures for the vision

functions investigated but reported the results of the study It was reported that all children had

normal visual acuity

In 2008 Alvarez and Puell 88 studied accommodation functions in school children with reading

difficulties in Madrid Spain Eighty seven poor readers (mean age 92 years) and 32 control

(mean age not indicated) participated in the study all in grades three through grade six

Relative accommodation amplitude of accommodation and accommodation facility was

assessed

45

The most current literature reviewed was reported by Bucci Gignac and Kapoula 89 in 2008

The authors studied poor binocular coordination of saccades in dyslexic children in France Of

particular interest to the current review is the assessment of visual acuity and refraction

(though was assessed under cycloplegia) heterophoria near point of convergence and fusional

vergence reserves The study participants comprised of 18 dyslexic children (mean age 114

years and mean IQ was 105) The control group comprised aged-matched non-dyslexic

children mean age 11 2 years There were no details on the method of assessment of visual

acuity and refraction but it was indicated that a cycloplegic refraction was conducted

However the authors reported that all children had normal visual acuity but the results for

refractive errors were not reported

In 1991 a study on stereopsis accommodative and vergence facility was conducted by

Buzzelli 6 in the United States of America Thirteen normal readers of average age thirteen

years three months and thirteen dyslexics average age of thirteen years four months

participated in the study Both groups were matched for gender age and intelligence quotient

The study design was comprehensive The Snellen chart was used to assess the visual acuity

Buzzelli6 found that the dyslexics did not perform worse than the control subjects in visual

acuity

Ygge Lennerstarnd Axelsson and Rydberg2 studied visual functions in a Swedish population

of dyslexic and normally reading children in 1993 Eighty six 9-year- old children matched to

controls with regard to age gender class in school and intelligence participated in the study

VA was assessed with ldquoordinary optotypes chartsrdquo They reported that the subjects from the

control group had a better visual acuity than the dyslexic group at both distance and near and

that the results showed statistically significant differences (distance at p =003) and (near at

p=0005)

A comparative study of dyslexic and normal readers using orthoptic assessment procedures

was conducted by Goulandris McIntyre Snowling Bethel and Lee90 The study was

conducted in 1998 in the United Kingdom and comprised 20 dyslexic and 20 chronological and

46

reading age-matched control participants A unique aspect of the study was that the distance

VA was assessed using the Cambridge Crowding Cards This test according to the authors

was devised to evaluate letter recognition acuity in conditions of lsquocrowdingrsquo when other letters

surround the target letter and that the test is designed to ensure that abnormalities of vision

which would not be revealed reliably using single targets were detected The near acuity was

assessed using the Snellenrsquos chart The use of different VA chart at distance and near may

raise a concern of standardization They reported that the prevalence of distance and near VA

(using a 69 criteria) was similar between the groups

In 1992 Evans Drasdo and Richards91 investigated optometric correlates of reading disability

in 10 children aged between eight and fifteen years who were referred to the optometry clinic

by the child psychologist The authors reported only the statistical values mean binocular

distance visual acuity of + 091 and mean binocular near visual acuity of + 089

47

23 Refractive Errors

In the study by Alvarez and Puell 88 no information was given on the protocol used to assess

refractive error but the refractive error results were reported For the poor readers mean

spherical equivalent refractive errors for the right and left eye were 020 06 and 020 06

respectively Fifty-seven (655) children were emmetropic In the control group mean

spherical equivalent refractive errors for the right and left eye were - 020 08 and - 014

080 respectively Twenty (645) children were emmetropic In contrast Evans et al 91

reported the mean spherical equivalent refractive error (by retinoscopy) of + 077DS for the

(right and left eye was similar)

231 Myopia

Myopia is characterized by a decrease in distance visual acuity Myopia would usually not be

expected to affect reading skills since students can make the necessary adjustments to their

reading positions However from a clinical standpoint unusually high myopia myopic

anisometropia and astigmatism could influence patients near performance and cause some

discomfort to the child There is no evidence in the literature that myopia is associated with

poor reading In the study by Alvarez and Puell88 57 of the children from the dyslexic

group were myopic compared to 194 from the control group

In 1987 Rosner and Rosner 93 conducted a retrospective comparison of visual characteristics of

data for two hundred and sixty one children with learning difficulties in the United States of

Ameria who had been identified by the schools as manifesting learning difficulties (LD) The

control group consisted of four hundred and ninety six children without learning difficulties

The subjects from both groups were of age range between six and twelve years The children

had been examined at the University of Houston Optometry Clinic The data collection

techniques used was not given but it may be assumed to be routine optometric eye examination

48

protocols because the study was conducted at the university optometry clinic The prevalence

of myopia (minus 050 dioptres) was reported to be 54 in the non-learning disabled patients

while 19 of the learning disabled patients were myopic

One limitation with this study however was that all subjects were patients of an eye clinic

where the two main reasons for referral are reduced visual acuity and school learning

problems Consequently the prevalence of visual defects in subjects selected from such a

clinical setting may have been higher than those from a non-clinical setting

In a review study conducted by Grisham and Simon82 the prevalence of myopia was lower in

the reading disabled group than among the normal readers Similarly Grosvenor 94 analysed

data from several studies and concluded that myopia is consistently associated with good

reading performance One limitation with this analysis however that is the conclusion drawn

from the review was based only on studies that found positive relationships between vision

anomalies and reading ability Grosvenor 94 however remarked that ldquomany authors failed to

find the relationship because they used ineffective methods

One of the earliest studies on the subject of vision and reading difficulty was conducted by

Eames 95 in 1948 in the United States of America He studied refractive and binocular vision

anomalies in one thousand (1000)rdquoreading failures one hundred and fifty (150) unselected

children and five hundred ophthalmic patientsrdquo Comparing the poor readers and the

unselected groups (control) he found the incidence of myopia to be same in both groups (4)

Myopia was defined as -100 diopter sphere There was no statistical analysis in that study and

the data collection techniques were vague although it was indicated that no cycloplegia was

used While the study may be criticized as not being current it is the major and early study to

give an insight into the subject of vision and reading disability

Eames95 and Shearer96 reported a similar prevalence of myopia (4) Shearer96 investigated

ldquoeye findings in children with reading difficultiesrdquo Two hundred and twenty two children

participated in the study The subjects were screened using the Keystone telebinoculars The

prevalence of myopia defined as - 050 dioptres sphere was 4 He concluded that vision

49

functions are not related to reading ability

232 Hyperopia

Hyperopia is often associated with poor reading performance probably through the mechanism

of deficient accommodation and poor motor fusion skills 82 A child with a relatively high

degree of farsightedness may not report blur vision at distance due to compensation for

hyperopia through the accommodation mechanism which also is used to create clear vision at

near This results in overuse of the accommodation system both optically and physiologically

and a subsequent fatigue or spasm of the accommodation system 97

One of the studies that classified refractive error in the literature was conducted by Rosner and

Rosner93 They reported that hyperopia appeared to be more prevalent (54) in learning

disabled than in the non-learning disabled children with a prevalence of 16

In the study conducted by Alvarez and Puell88 287 of the dyslexic group was hyperopic

compared to 161 from the control group Grisham and Simon82 in a literature analysis of

refractive errors and reading process reported that the prevalence of hyperopia among poor

readers is higher than among good readers and that the correction of hyperopia resulted in

improved performance

Grosvenor 94 reported that hyperopia is usually associated with poor reading Similarly in

Eamesrsquo study95 43 of poor readers had hyperopia while for the control group only 12 were

hyperopic The author gave no information on statistical analysis Shearer96 reported the

prevalence of hyperopia to be 16

233 Astigmatism

Clinically uncorrected moderate to high astigmatism can cause discomfort and lead to poor

reading performance at near distance Relatively few studies have attempted to study the

relationship between astigmatism and reading disability The prevalence of astigmatism

reported was higher in the dyslexic group compared to the control group in all the studies

reviewed 39395 These studies include the study by Latvala et al 3 who reported that 36 of

50

the dyslexic subjects were astigmatic (of greater than -100 diopter cylinder) and none from the

control group had astigmatism of that magnitude Eames95 reported the prevalence of

hyperopic astigmatism of more than -100 diopter cylinder among the poor readers to be 6

and 4 in the unselected group while the prevalence of myopic astigmatism was the same

(1) in both groups Rosner and Rosner 93 found a higher prevalence (30) of astigmatism

amongst the non-learning disabled subjects compared to the learning disabled group (27) In

Shearerrsquos 96 study of eye findings in children with reading difficulty the total prevalence of

astigmatism classified as myopic astigmatism (3) and hyperopic astigmatism (3) reported

was 6 There was no control group in this study In the study by Alvarez and Puell 88

astigmatism was detected in seven (8) and eight (92) right and left eyes respectively while

astigmatism was detected in five children (161) from the control group Ygge et al 2

reported that the prevalence of astigmatism was higher in the dyslexic group (28 left eye 25

) compared to the control groups (right eye 183 left eye 243) although there was no

statistically significant difference between the groups (p=025)

234 Amblyopia

Only two studies on the available literature consulted reported on the prevalence of amblyopia

both indicating a higher prevalence amongst the dyslexic group compared to the control group

In the study by Latvala et al 3 the prevalence of amblyopia in the dyslexic group was 36 and

none in the control group had amblyopia Although a detailed prevalence of refractive error

was not presented in that study it was reported that nearly all subjects had visual acuity of 07

In relating refractive error to amblyopia the prevalence of astigmatism in the control group

was 36 and none from the dyslexic group had astigmatism Consequently a low prevalence

of amblyopia may be related to fairly good visual acuities in both groups On the contrary

Rosner and Rosner 93 reported that 4 of the non-learning disabled patients had amblyopia

compared to the 3 found in the learning disabled patients Hyperopia is the refractive error

that is more often associated with amblyopia However this study presented a higher

prevalence of amblyopia (4) but a lower prevalence of hyperopia (16) in the non-learning

disabled patients However a difference of only 1 is not high enough to account for the

difference

51

235 Anisometropia

Anisometropia has been thought to be associated with poor reading skills probably through the

mechanism of poor sensory and motor fusion rather than reduced visual acuity It degrades

binocular coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 82

Three studies2395 reported findings on anisometropia Latvala et al 3 reported a higher

prevalence of anisometropia (of greater than or equal to one diopter) of 6 for the control

group than in the dyslexic group with 36 Similarly Ygge et al 2 found a higher prevalence

of anisometropia in the control group (158) than in the dyslexic group (94) Eames 95

reported that 13 of poor readers were anisometropic while 6 from the control group had

anisometropia According to Grisham and Simons 82 only few studies have considered

anisometropia to affect reading performance The authors concluded that there was insufficient

number of studies to draw firm conclusions from their review

Other studies2598 failed to classify refractive error according to the type These include

studies by Evans et al 5 who investigated some sensory and refractive factors in dyslexia The

distribution of refractive errors was found to be similar in both the dyslexic and control groups

The details of the participants profile was given earlier (section 21) The refractive errors

were assessed by distance retinoscopy and subjective refraction A cross cylinder was used to

measure astigmatism The spherical equivalent refraction was calculated by adding one-half

of the cylindrical components to the spherical component It was not indicated if refraction

was performed under cycloplegia Ygge et al 2 assessed refractive errors objectively using

streak retinoscopy under cycloplegia They reported that the distribution of refractive errors

were similar between the dyslexic and control groups

Sucher and Stewart 98 examined 136 fifth and sixth grade children comprising seventy two

with learning disability and 64 from the mainstream school as the control group The learning

52

disabled children were selected from a classroom of children attending special supervision and

instruction Children from both groups were matched by grade level No information on

intelligence quotient was given The testing protocol used to assess refractive error was not

indicated The authors reported a higher prevalence of refractive error in the dyslexic (208)

than in the control (94) group As indicated earlier refractive errors were not classified

according to the types in the three preceding studies 2598

Hoffman 83 reported the prevalence of refractive error to be 215 There was no report on the

control group and the techniques used to assess refractive errors were not indicated In the

study by Helveston et al 85 only 1 of the participants had refractive error when assessed

objectively with the retinoscope without the use of cycloplegia Refractive errors were not

classified according to the types (myopia hyperopia or astigmatism)

In 1991 Hall and Wick 99 conducted a study to investigate the relationship between ocular

functions and reading achievement in the United States of America One hundred and eleven

children (in grades one through grade six) participated in the study Eleven ocular functions

were investigated including visual acuity saccade eye movements accommodation facility

amplitude of accommodation accommodation posture heterophoria horizontal fixation

disparity vertical fixation disparity near point of convergence titmus and Randot stereopsis

Accommodation facility was measured using the flipper technique The amplitude of

accommodation was determined using the push up to blur technique Heterophoria fixation

disparity and near point of convergence were also measured but the techniques used were not

indicated The age range of the subjects studied was not indicated Using multivariate

correlation they found no statistically significant relationship between the ocular functions and

reading abilities as they noted that the acquisition of reading skills is given by many forces

which include the use of remedial instructions and language skills the role of peer pressure and

innate intelligence

53

24 Near Point of Convergence

Anomalies of convergence is one of the common problems encountered in the optometric

practice especially in the pediatric population and the reports on near point of convergence has

been presented comprehensively in the literatures reviewed

Grisham et al 86 measured the near point of convergence by asking the student if the clown face

target was seen as single when the examiner held it approximately 20 cm in front of the

studentrsquos eyes If so then the student was requested to say when the object appeared to double

as it moved closer to the face The examiner placed one end of an accommodative rule on the

studentrsquos forehead and judged where the two eyes diverged as the target moved toward the

studentrsquos face The measurement was repeated three times The authors found that 846 of

the students had near points at 8 cm or closer which they considered ldquonormalrdquo while 154

had near point of convergence of 9 cm or farther

In the study by Kapoula et al87 the NPC was determined by placing a small pen-light at

3040cm in the mid plane in front of the subject and moving it slowly towards the eyes until

one eye lost fixation The authors reported that the NPC was significantly more remote

(gt10cm) in dyslexics than in non-dyslexics (36 versus 15)

Bucci Gignac and Kapoula 89 followed similar protocol as in the study by conducted Kapoula

et al 87 The near point of convergence was determined by placing a small pen-light at 3040

cm in the mid plane in front of the subject and moving it slowly towards the eyes until one eye

lost fixation It was reported that the NPC was not statistically different between the two

populations the median value was 5 cm for both dyslexic and non-dyslexic children

Latvala et al3 failed to indicate the technique used in assessing the near point of convergence

54

but simply noted that ldquoconvergence ability was measuredrdquo but reported that convergence near

point greater than or equal to 8cm was found in 127 of the dyslexic subjects and 2 of the

control group The authors concluded that the near point of convergence was the only variable

that showed a statistically significant difference between the groups They concluded that

ophthalmic factors should not be overlooked as a contributing factor to dyslexia as they may

constitute part of the dyslexic syndrome and recommended that vision anomalies should be

corrected whenever detected

Shearerrsquos 96 findings are similar to the results reported by Latvala et al 3 He assessed the near

point of convergence using the keystone telebinoculars and reported that 12 of the

participants in his study had convergence problems

In a relatively elaborate study Evans Drasdo and Richards100 investigated accommodative and

binocular functions in dyslexia The subjects were 43 control and 39 dyslexics aged between

seven years six months and twelve years three months An intelligence quotient range of over

85 for the dyslexics and over 90 for the control with the Wechsler Intelligence Scale for

Children (Revised) (WISC-R) was recorded They measured the near point of convergence

using the royal air force (RAF) rule It was concluded that all subjects were able to report a

clear subjective break and recovery and reported no statistically significant difference between

the two groups in either the near point of convergence

Hoffman83 assessed convergence using keystone visual skills tests but grouped the results as

binocular coordination which makes it difficult to analyze and compare while in the study by

Helveston et al 85 98 of the participants had normal near point of convergence

In 2002 Keily Crewther and Crewther 12 investigated the relationship between functional

55

vision and learning to read in Australian school children Two hundred and eighty-four

children (mean age 99 18 years) received a vision screening emphasizing binocular

anomalies associated with discomfort at near (distance and near visual acuity distance vision

ldquochallenged with binocular +1 D lensesrdquo near heterophoria near point of convergence

stereopsis and accommodative facility) Children were classified as normal readers (n = 195)

children with dyslexia (n = 49) or learning disabled children (children who have learning

difficulty with impaired intelligence quotient) (n = 40) It was reported that the near point of

convergence were better in the dyslexic group compared to the normal and the learning

disabled group but found no statistically significant difference between the groups In

constrast Evans et al 91 assessed near point of convergence using the RAF rule and reported a

highly statistically significant relationship between NPC and reading retardation (p=0019)

25 Heterophoria

Heterophoria is a common binocular problem and in clinical practice it is particularly

important when analyzed in conjunction with fusional reserve as it gives an indication of the

zone of comfortable binocular vision

The distance phoria is a clinical measurement of the tendency of the two eyes to deviate from

parallelism when viewing a target usually at six meters When a closer viewing target is

utilized (say at about the individuals reading distance) the resulting near phoria measures the

degree to which the eyes over converges (esophoria) or underconverges (exophoria) to the

plane of the target97 This latent deviation requires fusional vergence to maintain single

binocular vision and at near they involve the accommodation system Smooth functioning of

these systems particularly at near is important for near tasks92

Several studies312858789959698100101 have attempted to investigate the role of heterophorias on

reading ability and all reported remarkably varying findings The assessment of heterophoria

was extensive in the study conducted by Latvala et al 3 Heterophoria at distance was evaluated

56

using the cover test and Maddox rod The prism bars was used to quantify their findings The

Maddox wing was used to assess phoria at near At near they reported a higher prevalence of

esophoria (61) among the control than the dyslexic group (58) a higher prevalence of

exophoria (25) among dyslexics than the control group (122) and a higher prevalence of

vertical phoria (61) among the control than the dyslexic subjects (36) At far distance

they found a higher incidence of esophoria (113) among dyslexics than the control (82)

subjects a higher prevalence of exophoria (2) among the control group than among the

dyslexics (19) and vertical phoria to be more prevalent in the control (41) than in the

dyslexics (38)

Keily et al 12 assessed heterophoria using the Prentice card and reported that there was a slight

but insignificant tendency for the dyslexic subjects to be more exophoric than the normal and

those with learning disability but found no statistically significant difference The test distance

was not indicated

Evans et al 100 in a study of accommodative and binocular functions in 43 control and 39

dyslexics reported that all subjects examined were orthophoric at distance with cover test

except for one dyslexic subject who demonstrated 10 prism diopters right esotropia and two

diopters right hypertropia which changed to 15 prism diopters right esotropia with two prism

dioptre right hypertropia at near At near horizontal phoria results were similar in both groups

Evans Efron and Hodge 101 studied the incidence of lateral phoria in 45 children with specific

learning disability in the United States of America in 1977 The participants were selected from

a psychologistrsquos file The control group comprised of 364 children in second third and fifth

grades attending regular school It was not indicated why there was such a big difference in

the number of participants from both groups The test used in assessing phoria was the

ldquoRocket card which was used in conjunction with red- green spectaclesrdquo They reported a

57

prevalence rate of 1135 in the control group while 222 of the children from the specific

learning difficulty group had heterophoria There was no statistically significant difference

between the groups In concluding it was recommended that phoria conditions may not cause

reading or learning disability but that the presence of such conditions could contribute to visual

perceptual and attention abnormalities

A report by Sucher and Stewart98 contained limited information as the authors did not detail

whether a higher prevalence rate among control (105) than dyslexic (86) was for near or

distance phoria Similarly Eames95 failed to indicate the distance at which the measurement

was made but reported that 33 of poor readers had exophoria and 22 of control were

exophoric but did not indicate whether distance or near

Helveston et al 85 used the Wottring amblyoscope to determine ldquoobjective and subjective angle

of ocular alignmentPrime and reported that 96 of the participants in their study were orthophoric85 The criteria used to classify ocular deviation were not indicated

Another comprehensive report of heterophoria was documented by Shearer96 The subjects

were screened using the Keystone telebinoculars and reported that 1 of the study participants

had vertical phoria (gt1 prism diopter) 2 had esophoria (gt 4 prism diopter) 3 had

exophoria (gt 4 prism diopter) 26 had exophoria at near It was concluded that vision

functions are not related to reading ability Hoffman 83 reported that ninety three (93) subjects

(869) had problems of binocular coordination but failed to indicate whether it was phoria or

tropia

Kapoula et al 87 evaluated ocular alignment using the cover test and reported that at far

distance the median value of phoria was zero for both non-dyslexic and dyslexic children (p =

019) At near the median value of phoria was similar between the two groups (minus2 prism

58

diopters for non-dyslexics and minus15 prism diopters for the dyslexics (p = 019)

Bucci et al89 measured heterophoria at near and far using the cover-uncover test As

documented by the authors exophoria was not statistically different between the two groups (p

= 02) the median was ldquo0rdquo prism diopters for dyslexics and 2 prism diopters for non-dyslexics

No information was given on other aspects of heterophoria

Simons and Grisham92 conducted a literature analysis on binocular vision anomalies and

reading emphasizing studies with positive and negative relationship between binocular vision

and reading The authors concluded that binocular conditions such as esophoria exophoria

restricted ranges of fusion foveal suppression accommodative insufficiency and oculo-motor

disorders at reading range are more prevalent among poor readers than normal readers

In another study Ygge Lennerstarnd Rydberg Wijecoon and Petterson102 investigated

oculomotor functions in a Swedish population of dyslexic and normally reading children in

1993 The study comprised 86 nine -year- old dyslexic children matched to control with regard

to age gender and intelligence Heterophoria was measured using the cover test They

reported that for the dyslexic group ldquo23 pupils exhibited a phoria (264 9 eso- and 14 exo-

phoria)rdquo whereas in the control group ldquo20 pupils had a phoria (232 6 eso- and 14 exo-

phoria)rdquo and that the difference was not statistically significant (p = 034) At near the

incidence of exophoria was 437 for the dyslexic group and 453 in the control group

There was no statistically significant difference (p=055)

26 Strabismus

Several authors34 83-85 90 93102 included the measurement of strabismus in their studies

reporting remarkably varying findings Helveston et al 85 assessed ocular alignment using the

Wottring amblyoscope and reported that only between 2 - 4 of the students had phoria or

59

tropia of more than four prism dioptres Latvala et al 3 reported the prevalence of tropia at near

for the dyslexic group to be 36 and 2 for the control group Hoffman83 assessed ocular

alignment using the keystone visual skills test and reported that 13 subjects (9 exotropes and 4

esotropes) (1214) had strabismus It is difficult to comment if this relatively high

prevalence of strabismus is related to hyperopia as the refractive error prevalence of 215 was

not categorized as to the type In a study by Sherman84 a total of four children (8) were

strabismic classified as three exotropia (alternating) and one esotropia (alternating) Rosner

and Rosner93 in their retrospective analysis reported that in the learning disabled group 51

had esotropia 39 had exotropia while in the non- learning disabled group 8 were

strabismic It was not unexpected to find a higher prevalence of strabismus in this

retrospective study Aasved 4 found that the dyslexics had a higher prevalence of manifest and

latent convergent strabismus than the control subjects

Goulandris et al90 assessed strabismus using the cover test and reported that there was no

statistically significant difference between the two groups in the prevalence of strabismus

Ygge et al 102 reported that at distance manifest strabismus was more frequent among the

dyslexic (8) pupils than the controls (23) but that the difference was not statistically

significant At near the prevalence of manifest strabismus was 8 (all eso-) for the dyslexics

and 35 (all eso-) in the control group

27 Accommodation Functions

271 Amplitude of Accommodation

The accommodation mechanism is extremely important for learning Children who suffer

some anomalies of accommodation are more prone to fatigue quickly and become inattentive

than those who have normal accommodation function According to Flax 22 an ldquoinefficient

accommodation function is a significant contributor to lowered achievement in the upper

grade

60

The findings on various aspects of accommodation functions were diverse Metsing and

Ferreira 103 studied visual deficiencies in children from mainstream and learning disabled

schools in Johannesburg South Africa in 2008 One hundred and twelve (112) children from

two learning disabled schools and eighty (80) children from a mainstream school in Johannes-

burg participated in the study The vision functions examined were visual acuity refractive

status ocular health status accommodative functions (Posture facility and amplitude)

binocularity (cover test vergence facility smooth vergences NPC and ocular motilities (direct

observation) However according to the authors only the results for accommodation

accuracy amplitude of accommodation vergence facility and saccadic accuracy are reported

since only these variables showed statistically significant relationship on the relationship

between the (mainstream and learning disabled) groups Metsing and Ferreira 103 reported

that a high percentage (516 and 533) of low amplitude of accommodation for the right

and left eyes respectively was found in the mainstream group compared to the 291 and

288 in the learning disabled group The relationships between the mainstream group and

reduced amplitudes of accommodation of the right (p=004) and left eyes (p=0001) were

found to be statistically significant (plt005) The relationship between the mainstream group

and reduced amplitude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to

be moderate and that between the mainstream group and the right eyes (Cramerrsquos V= 0286) to

be low

In the study by Grisham et al86 accommodative amplitude was assessed using the

accommodative rule ldquoThe student held an occluder over the left eye The examiner asked if

the student could see the (2030) target at the larger end of the Occlud-A Measure held at about

20 cm from the face If yes then the student was asked whether the target was in focus or ldquonot

blurryrdquo If the student responded that the target was in focus then the target was moved at

approximately 2cm per second toward the studentrsquos face and the student was asked to report

when the target first became blurryrdquo About 638 of the participants had amplitude of

accommodation of 11D which the authors considered equivalent to the expected amplitude of

61

accommodation for the age About 247 had inadequate accommodation amplitude while

115 had ldquoborderlinerdquo amplitude of accommodation

In the study by Evans et al 100 the amplitude of accommodation was measured using the push up

method They reported that the amplitude of accommodation was significantly reduced in the

dyslexic group and that the mean accommodation lag was calculated for each eye but found no

statistically significant difference between the two groups

Alvarez et al 88 assessed monocular accommodative amplitude using the minus lenses method

The subject viewed horizontal line of 2030 letters at 33cm as the examiner introduced minus

lenses in 025D increments until the target first becomes blurred According to the authors the

working distance adjustment used was kept at 250 DS to compensate for minification The

authors reported that monocular accommodative amplitude was significantly lower (p lt 0001)

in the group of poor readers (right eye 91 D 23 left eye 90 D 23) than in the control

group (right eye 105 D 17 left eye 105 D 17)

Helveston et al85 measured the amplitude of accommodation using the push up to blur

technique and reported that ldquoan ability to accommodate more than eleven dioptres was found in

between 90 and 93 of the studentsrdquo Ygge et al 102 assessed amplitude of accommodation

using the RAF rule and reported that the two groups performed similarly A similar technique

was used by Goulandris et al 90 and a lack of statistically significant difference between the

groups was reported

272 Accommodation Facility

In the same study Evans et al 100 reported that the dyslexic group appeared to be slower at a

62

test of accommodative facility The authors discredited the flipper technique as the

unnatural act of altering accommodation without changing vergence They further noted that

the flipper testhellip would still imperfectly reflect the normal visual environment owing to the

absence of a stimulus to vergence and accommodation

Grisham et al86 measured accommodative facility using the 200D lenses flipper technique

and the technique was performed as follows The 2D lenses was flipped each time the

student said the target was ldquoclearrdquo The target for this test was the 2030 target used for

accommodative amplitude The left eye was occluded The student was asked whether the

target was clear when held at about 40 cm If so then the student was told that the test would

be timed and that it should be reported whenever the target was ldquoclearrdquo The examiner then

started a stopwatch and counted the number of times the lenses were flipped in 30 seconds

That number was taken to represent the cycles per minute (cpm) and therefore to represent the

studentrsquos ability to change accommodation over time Based on the criteria used in their study

the authors reported that 76 of the students had adequate monocular accommodative facility

while 236 had ldquoweak accommodative facility of 9 cpm or less and 81 had accommodative

facility of as low as 1 or 0 cpm

In the study conducted by Alvarez et al 88 accommodative facility testing was conducted using

the following procedure The subject wearing his or her habitual correction was seated and

positioned so that the spectacle plane was 40cm from the Bernell Acuity Suppression Slide

(VO9) A 200 D accommodative demand was used with a 2030 test target at 40cm The

patient was asked to view the reduced Snellenrsquos letters 2030 in line 5 and say ldquonowrdquo when the

letters in that line appear clear and single and to report letters missing from lines 4 and 6

during the binocular testing They reported that the binocular accommodative facility values

were significantly lower (p lt 005) in the poor readers group (49 cpm 31) than the control

group (63 cpm 29)

63

Sucher and Stewart 98 assessed binocular accommodative facility at 35 cm using 150 diopter

flippers lens for ldquoone minuterdquo and ldquoused the University of California Berkeley clinic standard

of 20 cycles90s as a ldquocriterion for successrdquo found a higher prevalence of accommodative

infacility in dyslexics (222) than the control group (86)

On the contrary three authors6 12 93 found the control subjects to have better accommodation

facility than the dyslexic subjects The studies are Rosner and Rosner93 found a higher

prevalence of accommodative infacility among the control (10) than the learning disabled

group (9) Similarly Buzzelli6 found that the dyslexic subjects showed better

accommodation facility than the control group To assess accommodation facility ldquopatients

were asked to clear the number 9 on the Bernell Vectogramm Target at 40cm hellip alternately

viewed the target through a + 200DS and a ndash 200 DS lenshellip for 20 cycles) There was no

statistically significant relationship between the two groups

Keily et al12 assessed accommodative facility using +200DS and ndash 200DS flippers lenses

with an N6 fixation target for one minute at 30 cm rather than 40 cm because according to the

authors the children were required to hold the reading material They reported that

accommodative facility was better in normal readers than either of the poor reading groups

In another study Evans and Drasdo104 reviewed ophthalmic factors in dyslexia and commented

that convergence insufficiency accommodative dysfunctions and unstable eye movements are

more common in dyslexic population than in normal readers

Latvala3 simply noted that some aspects of accommodation were difficult to measure and failed

to give any details

64

273 Relative Accommodation

Alvarez and Puell88 measured relative accommodation using a phoropter The subject viewed a

horizontal line of 2030 letters at 40cm with hisher best refractive correction The examiner

introduced minus or plus spherical lenses in 025 D steps binocularly until the patient reported

first sustained blur Negative and positive relative accommodation values were similar in both

groups This was the only study accessed that reported on relative accommodation

274 Accommodation Posture

Evans et al100 assessed accommodation lag using the monocular estimate method of retinosocpy

and reported no statistically significant difference between the dyslexic and the control groups

Metsing and Ferreira103 measured the accuracy of the accommodative response (lag or lead)

objectively using the MEM retinoscopy in normal room illumination and found that the subjects

from the mainstream school demonstrated a high prevalence of lead of accommodation (40

and 405 for the right and left eyes respectively) However in the learning disabled group a

high prevalence of lag of accommodation (98 and 119 for the right and left eyes

respectively) was found The relationships were found to be statistically significant (p lt 005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000)

Sherman84 assessed accommodation amplitude and relative accommodation using minus lenses

and accommodative facility using the flipper techniques The lens powers for the plus and

minus lenses used to assess accommodation functions were not indicated His findings may be

difficult to utilize due to the somewhat unique way of classifying the results as ldquomechanical

problemsrdquo

Flax22 suggested further investigations in the nature of accommodative function in reading

65

disabled children indicating that the use of the push-up method of assessing the amplitude of

accommodation is unreliable due to its lack of sensitivity Flax22 reiterated that

accommodation has a contribution in learning disabilities and that accommodative function

inefficiencies tend to play an increasing role as the youngster moves through school and that

inefficient accommodation function is a significant contributor to lowered achievement in the

upper grade

28 Vergence Facility

Vergences are a clinical measure of an individualrsquos ability to overcome the prismatically

induced optical displacements of a target in space Convergence relates to each eye fixating

closer in space than the real physical location of the target while divergence is the measure of

the eyes to fixate further in space while still maintaining single binocular vision97

Buzzelli 6 assessed vergence facility as follows ldquopatients were asked to align a vertical picture

(box-X-O) slide made of anaglyph material for monocular presentation under binocular

conditions The target measured 21 mm in the vertical meridian and 10mm in the horizontal

meridian The subjects alternately viewed the target through 16 prism base out and 4 prism

base in The test was continued for 20 cyclesrdquo The author reported that the dyslexics

completed the vergence eye movement task significantly slower (300 seconds) than the normal

readers (240 seconds) A statistically significant relationship between the groups was reported

(p lt 005) It was recommended that the possible role of vergence in dyslexia should be

investigated on a larger sample of dyslexic and control groups

Metsing and Ferreira103 assessed vergence facility test at near (40 cm) with the subjects

presented with a vertical row of letters (69) using the 8 PD (BI) and 8 PD (BO) mounted in a

flipper device They reported that the prevalence of poor vergence facility (205) was found

to be higher in the mainstream group compared to the learning disabled group (183) The

relationship between the mainstream group and poor vergence facility (p = 0000) was found

66

to be statistically significant (plt005) A medium to strong relationship was found to exist

between the mainstream group and poor vergence facility (Cramerrsquos V = 0369) It was

concluded that children from mainstream schools are likely to present with poor vergence

facility compared to children from schools of the learning disabled

2 9 Fusional Reserves

The compensatory effect of fusional vergence reserves on phoria is very important as difficulty

with reading is likely to occur when there is uncompensated phoria in conjunction with

reduced fusional vergence reserves 92

A major study was reported by Evans et al 100 which found that negative and positive reserves

were significantly reduced in the dyslexic group relative to the control The ldquoprism vergence

amplitude calculated as the difference between base out and base in break and recovery points

was relatively reduced in the dyslexic grouprdquo They remarked however that several subjects

were unable to appreciate a subjective blur point but the results for the break and recovery

were complete

In the study conducted by Latvala et al 3 the incidence of fusional amplitude greater than or

equal to thirty two prism dioptre at near was higher in the dyslexic (75) than in the control

(61) group At distance they found fusional amplitude greater than or equal to15 prism

dioptre to be higher in the control (122) than in the dyslexic (94) group

Disordered vergence control was studied by Stein Riddell and Fowler 105 in the Uniteed

Kingdom in 1988 The subjects were thirty nine dyslexic children aged eight to eleven years

and a control group of twenty subjects of similar age group The vergence control was

measured using the Synoptophore They reported that two thirds of the dyslexic group had

67

abnormal vergence control According to the authors vergence eye movements never seem to

have been investigated in detail during reading before and that the study was the first in which

eye movement recording have been used to demonstrate differences between dyslexics and

normal readers

According to Grisham et al86 ldquoConvergence and divergence break and recoveryrdquo were

measured at 40 cm with a horizontal prism bar The student was first asked if the clown face

target on the smaller end of a standard optometric measure (Occlud-A-Measure Bernell

Mishawaka Indiana) could be detected If so then the student was requested to say whether

the target appeared single or double Once single vision was obtained the bar was moved

relative to the studentrsquos eye until double images were reported That point was considered

ldquobreakrdquo Then the motion of the bar was reversed until the student reported the image single

and this point was considered ldquorecoveryrdquo This procedure was first performed for base in (BI)

position of the bar over the left eye and repeated with base out (BO) position over the right

eye The authors reported that a large number of students had poor convergence skills 38

break at less than 18 PD and 95 recover at 7 PD or less These values were considered to be

in the ldquoweakrdquo to ldquovery weakrdquo range and remarked that in contrast to convergence lsquomodalrdquo

values for break (14 PD) and recovery (10 PD) were not in the ldquoadequaterdquo range but would be

classified as ldquovery weakrdquo for break and ldquoweakrdquo for recovery As a result very high

percentages of students had poor divergence skills 82 break at less than 20 PD and 60

recover at less than 11 PD

Kapoula et al 87 measured vergence reserves using prism bars at far and near distance for both

groups They reported that the median value for positive fusional vergence was 16 prism

diopters and 14 prism diopters for non-dyslexics and dyslexics respectively at far distance (p

= 012) and 20 prism diopters for both non-dyslexics and dyslexics at near distance For

negative fusional vergence they reported that ldquodivergence amplituderdquo was significantly

different in the two groups for both distances (p lt 0005) At far the median value was 6

68

prism diopters and 4prism diopters in non-dyslexics and dyslexics respectively and 12 prism

diopters and 10 prism diopters respectively at near

Bucci et al 89 measured fusional reserves at far and near using prism bars (base in and base

out) According to the authors orthoptic evaluation of vergence fusion capability showed a

significantly limited divergence capability for dyslexics compared to non-dyslexics the

median value was 10 prism diopters for dyslexics versus 17 prism diopters for non-dyslexics

In contrast convergence amplitude was in the normal range for the two populations while for

dyslexics the amplitude was significantly larger than for non-dyslexics the median value was

30 prism diopters and 18 prism diopters for dyslexic and non-dyslexic children respectively

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups

The mean fusional convergence capacities at distance were 1680 prism diopters for both the

control and the dyslexic group At near the corresponding figures were 2640 prism diopters

and 267 prism diopters respectively The mean fusional divergence capacity at distance was

650 and 620 prism dopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 105 and 102 prism diopters respectively There was no

statistically significant difference between the groups

210 Ocular Pathology

Ocular pathologies do not seem to be prevalent in the dyslexic population and it was not

reported on in the literature reviewed However according to Evans and Drasdo100 the

perspective to relate ocular pathology to dyslexia is in individuals who were normal readers but

developed reading disability due to pathological conditions such as stroke or tumors affecting

the right hemisphere These conditions may be relevant to the considerations of establishing the

relationship between ophthalmic problems and dyslexia100

69

Only one study83 included this as part of the investigation Hoffman83 reported that one child

(093) of the population of learning disabled children examined had acute conjunctivitis

A major study on dyslexic subjects (although outside the cope of the present study but worth

citing) was conducted by Raju11 in 1997 She studied oculomotor control in dyslexia and

reported that sixty five percent of the normal readers exhibited normal oculomotor control as

compared to the eleven percent of dyslexics and that fifty four percent of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills whilst twelve

percent of the control subjects displayed the same

70

LITERATURE REVIEW TABLE pages 70-72

71

72

73

211 Summary

Research into visual aspects of dyslexia has been undertaken mainly by optometrists and

ophthalmologists The main area of controversy is that ophthalmologists tend to deny any

relationship between vision and reading ability while optometrists assert that identifying and

correcting visual defects are an important part of managing dyslexia The findings of studies that

have investigated vision function in dyslexic population since the 1940s are inconclusive and

reflect the complexities associated with identifying the causes of dyslexic as well as its diagnosis

treatment and management

A range of study designs were used and the inclusion of a control group for example depended

on the objective of the study Some authors either compared the prevalence of vision defects in a

population of reading disabled group to the prevalence of similar vision defects in a group of

normal readers or only presented the prevalence of a particular visual condition in a group of

reading disabled children All the studies used convenience sampling methods at schools or by

including learners who were referred to their private practices the size being determined by the

number of dyslexic students available The main areas of vision functions studied were visual

acuity refraction binocular vision and ocular pathology The study findings were inconclusive

and the only vision variables consistently report ed across the studies was the association of

hyperopia with poor reading performance

The differences in results reported by the authors may be due to the methodological problems

such as differences in instrumentation and techniques failurecut-off criteria method of data

analysis lack of comparison group and subjectsrsquo selection from clinical population There was a

noticeable absence of studies on African children research has shown that African persons are

less at risk of myopia and more at risk for open-angle glaucoma than Caucasians There is

therefore scope for continued research among African children to determine whether there is any

relationship between dyslexia and vision

Research on dyslexia and vision that requires an interdisciplinary approach may yield different

results due to different approaches by different professionals However it is challenging to

74

design a study that would control for all variables possibly influencing reading ability and vision

function Reading performance is an extremely complex task and its measurement can be

influenced by many factors

Having reviewed the relevant literature on dyslexia and vision Chapter three will present the

research design the study population sampling procedure methods of data collection and

analysis

75

CHAPTER THREE METHODOLOGY

31 Introduction

This chapter will present the research design the study population sampling procedure

methods of data collection as well as method of data analysis

32 Research Design

The study was designed to provide empirical information to enable a comparison of the visual

characteristics of dyslexic children and normal readers A matched paired case-control

method was adopted Data was collected from both the dyslexic and control groups and

analysed

321 Sampling Design

Participants for both samples were selected using the convenient sampling method (based on

available subjects) and an optometric eye examination was conducted

322 Study Population and Participants

The study population comprised children attending a school for children with learning

difficulties from which the dyslexic children were selected to participate in the study while

the control group consisted of learners from a mainstream Durban school

i Dyslexic Group Participants for the study from the dyslexic group consisted of 31

African dyslexic children (the experimental group) selected from Khulangolwazi Special

School for children with learning difficulties in Clairwood South of Durban Psycho-

educational evaluationdiagnosis of dyslexia was not part of this study and was not

76

performed The school principal allowed access to all learnersrsquo file from which the

dyslexic learners where selected based on the school psychologistrsquos diagnosis Learners

attending this special school were referred from mainsream schools around Durban The

final diagnosis as to the type of learning difficulties was done by the school psychologist

None of the children were on any medication

The mean age for the dyslexic participants was 13 years and the learners were in grades

four through grade seven considering the fact that they were children that were reading two

grades behind their chronological age There were 15 boys and 16 girls Due to limited

number of learners who were classified as dyslexic it was difficult to recruit the targeted

number of one hundred participants for the study The subjects from both groups were

lsquoAfricanrsquo South Africans of African origin

ii Control Group Participants for the control group consisted of 31 children from a

mainstream school in Durban (Addington Primary School) The mean age of the children

was 11 years and 9 months A total of 15 boys and 16 girls were chosen ranging from grade

four to grade seven

323 Inclusion Criteria for the Dyslexic group

To be included in the study the subjects had to meet the following criteria

1 An average or above average intelligence quotient For this study an average was taken

as between 95 and above as recorded on the learners file by psychologist

2 Be two grades or more below grade equivalent in a mainstream school

3 The child has not been absent from school for more than 10 of the attendance days

The information on the learnersrsquo attendance was supplied by the school principal

324 Exclusion Criteria for the Dyslexic group

The following criteria resulted in children being excluded from the study

1 Presence of any emotional problems (information from the learnersrsquo file)

2 Presence of any systemic condition

77

3 Use of any systemic medication

The inclusion and exclusion criteria for the control group were similar to the experimental

group except that the children did not have any reading problems and were attending a

mainstream school

33 Ethical Clearance

The study protocol was approved by the then University of Durban-Westville research ethics

committee A letter of request for access to the schools was sent to the Department of

Education (Appendix B) Written informed consent (Appendix C) for access to the schools (for

both groups) was obtained from the Department of Education Written informed consents were

also obtained from the principals of the schools (mainstream and special school) (Appendix D)

Due to the difficulty in reaching the parents the school principals consented on behalf of the

parents for the learners to participate in the study The participants were fully informed of the

purpose of the study and accepted to co-operate with eye examination procedures

34 Testing Protocol

The testing was done first with the dyslexic learners and then at the mainstream school

35 Study Setting

The same testing arrangement was used for both groups A testing room was set up in one of

the offices provided by the school principal The distance acuity chart was placed at a point

six meters away from where the subjects sat and was used for the distance acuity chart and all

distance testing The examinations were done under room illumination except for some

procedures such as retinoscopy that required dim illumination resulting in the room being

dimmed

78

All testing was conducted in the morning as it was anticipated that better responses could be

obtained when the children were not tired Each school principal provided an assistant who

helped with various activities such as controlling room illumination The rationale and

technique for every procedure was fully explained to each participant and a trial reading was

taken to ensure that all instructions given were understood During the tests subjects were

asked (from time to time) if they were tired For any answer to the affirmative the testing was

discontinued and the child was allowed to have a break As a routine a break of 10 minutes

throughout the duration of entire procedure was allowed

The testing instruments were provided by the International Center for Eye Care Education

(ICEE) As the instruments were also being used by other researchers they were unavailable

on certain days resulting in some tests being postponed until a later date The data collection

took an average of two months per school All data were collected by the examiner alone and

each examination took an average of thirty minutes to complete

36 Data Collection Techniques

All tests were conducted in free space The data collection techniques used in the present

study was similar to techniques used in other studies 93100106-140 based on pediatric

populations (Table 31)

Table 31 Data Collection Tools and Measurement Outcome

InstrumentationTechnique Measurement Outcome

Bailey ndashLovie LogMAR Charts Visual Acuity

Royal Air Force (RAF) Rule NPC Amp of Accommodation

Welch Allyn Streak Retinoscope Objective Refraction

Trial Frame and Trial lenses Subjective Refraction

Cover Test - Occluder and Prism Bar Ocular Alignment

Maddox Rodpen torchPrism bar Heterophoria and Tropia

Maddox Wing Near Phoria

Prism Bar Heterophoria and Tropia

79

Pen torch PD rule Cornea reflex test-strabismus

Retinoscope and neutralizing lenses Accommodation Posture

Jackson cross cylinder Refine subjective findings

Trial Frame and neutralizing lenses Relative accommodation

+-2 flipper lenses Accommodation Facility

Direct Ophthalmoscope (Welch Allyn) Ocular Health

Prism Bars Fusional Reserves

361 Visual Acuity

Visual acuity is a measure of the acuteness or clearness of vision It assesses the ability of the

visual system to resolve detail which is dependent on the sharpness of the retinal focus within the

eye the sensitivity of the nervous elements and the interpretative faculty of the brain 106 Visual

acuity was assessed using the Logrithm of Minimun Angle of Resolution (LogMar) chart which

means that the letters change in size from line to line in equal steps of the log of the minimum

angle of resolution (MAR) Visual acuity was assessed monocularly and binocularly for both

distance and near The LogMar chart facilitates algebraic operations for statistical analysis and

allows for precise quantitative assessment of visual acuity It is recommended for use in research

studies in which visual acuity is a dependent variable106

Each subject was comfortably seated and the test was conducted with the examiner sitting in front

in such a way that the subjectrsquos view was not obstructed For monocular visual acuity the

subject was instructed to cover one eye with their right palm and to read the letters on the chart

This procedure was repeated binocularly and the test was done under normal room illumination 106 No subjects wore spectacles so visual acuity for aided vision was not indicated

362 Refractive Error

Refractive error was assessed objectively using the Streak Retinoscope (Welch Allyn) with a

+150D fogging lens (for an armrsquos length of approximately 67cm) with the subject fixating a

660 (to maintain fixation) optotype on the distance visual acuity chart As the subject focused

80

on the distance target on the chart the retinoscope light was passed across the subjects eye

(right hand used to scope right eye and left hand used to scope left eye) The motions

observed were neutralized with the appropriate lenses plus lenses for lsquowith motion and minus

lenses for against motion The subjective refraction was performed through monocular

fogging refined using the Jacksonrsquos cross cylinder followed by binocular balancing 106 The

retinoscopic results were used for analysis

Although retinoscopy was performed without cycloplegia as in other studies 395107108

cycloplegic refraction was not performed because the researcher (an optometrist) was not

licensed to use therapeutic drugs when the data was collected However it has to be

acknowledged that a non-cycloplegic examination among children has limitations among

those with hyperopia as the full extent of the hyperopia may be masked

363 Near Point of Convergence

The NPC was measured using the Royal Air force (RAF) rule The subject was instructed as

follows ldquoThis test measures your ability to turn your eyes in towards your nose Look

directly at the dot on middle of the line The image may appear to be blurring (not clear)

That is okay However if the dot becomes two say lsquotworsquo I will then pull the target away If

the dot becomes one again say lsquoonersquo rdquo The subject was asked to fixate on the dot on the

middle of the line as the target was advanced towards him The objective reading was taken at

the point when one eye loses fixation while the subjective reading was noted when the subject

reported the target to be double At a point when the subject reported double the target was

moved back until a point when the target was reported to be single again this was taken as the

recovery point The final break and recovery points recorded were the average of three tests

measurements The measurement was taken three times in order to detect fatigue which may

indicate poor convergence 111-113 The objective reading was taken for analysis

364 Strabismus

Strabismus was assessed using the Hirschberg test A penlight was held about 50cm from the

childs face in the mid plane and they were encouraged to fixate the light with both eyes open

81

The location of the corneal reflection on each cornea relative to the centre of the pupil was

then noted The displacement of the corneal reflection from the centre of the pupil (angle

lambda) was estimated in millimeters An ocular alignment was observed as a symmetric

displacement of the reflexes 114115 The examiner proceeded with the Kappa Test which is a

test of monocular fixation and is done while the child was monocularly viewing the examiners

penlight The penlight was maintained at 50 cm distance from the face The purpose of this

test was to determine the visual axis of each eye The examiner observed the location of the

corneal reflexes relative to the center of the pupil in the right eye and then in the left eye A

nasal displacement is signified by a (+) sign and a temporal displacement is signified by a (-)

sign with the amount quantified in millimeters away from center Again a normal location

will likely be either centered or +05 mm nasally displaced 114115

The interpretation of the magnitude and the type of deviation requires the data gathered from

both the Hirschberg tests The fixating eye is the eye in which the corneal reflex location is

the same on the Hirschberg and Kappa tests The strabismic eye is the eye in which there was

a difference The change in location from the Kappa tests as compared to the Hirschberg test

equals the amount and type of deviation Every 10 mm of corneal displacement is

approximately equivalent to 22 prism diopters A nasal change in location on Hirschberg

testing from the visual axis denoted by the Kappa test denotes an exotropia A temporal

change in location on Hirschberg testing from the visual axis denoted by the Kappa test

denotes an esotropia 114

365 Heterophoria

Ocular alignment was assessed using the cover test at six meters and 40cm for distance and

near respectively It is an objective technique and is very suitable for assessing ocular

alignment in children

For distance cover test the test target was a letter from the line above the subjectrsquos best visual

acuity of the worst eye The subject was advised that the muscle balance of their eyes was

assessed and that he should look steadily at the fixation target (an isolated letter on the chart)

at all times and ignore the occluder The coveruncover (unilateral) cover test was performed

82

to determine the presence of phoria or tropia 106114-115 To perform the unilateral cover test

the occluder was placed in front of the right eye held for approximately one to two seconds

and the left eye was observed for any movement If no tropia or phoria present the left eye

was covered and the right eye then observed Any movement observed was neutralized using

a loose prism bar in the corresponding base direction Base-in prism for exo deviation and

base-out for eso deviation

The alternating cover test was performed to assess the direction and magnitude of phoria or

tropia (it measures the total angle of deviation) 106114-115 The test conditions were the same as

the unilateral cover test The subject was comfortably seated and instructed to look at the

fixation target at distance The occluder was placed in front of the patientrsquos right eye held for

2-3 seconds and then quickly placed on the left eye The test was repeated several times with

the occluder held in front of each eye before quickly moving to the other eye while observing

the eye that was just uncovered Any deviation was neutralized accordingly 115 The Maddox

Wing was also used to assess near phoria under normal room illumination The technique was

assessed with the subject wearing his full subjective correction 115 The use of cover test for

the assessment of ocular alignment at both distance and near may have allowed for continuity

and standardization However the cover test and the Maddox wing were used by Evans et al 100 in the same study The subject was directed to look through the horizontal slits to view the

chart that comprised of a horizontal and vertical scale and a horizontal and vertical arrow The

construct of the Maddox wing is such that the right eye sees only the arrows while the left eye

sees only the scale It achieves dissociation by presenting independent objects to the patient

As the images are dissimilar the incentive to fusion is abolished and the eyes adopt the fusion

free position 115 The arrows are positioned at zero on the scales but through dissociation any

phoria will be indicated by an apparent movement of the arrow along the scale

To measure horizontal phoria the subject was asked ldquowhich white number does the white

arrow point tordquo The number on the scale represents the magnitude of the deviation and the

direction To measure vertical phoria the subject was asked to indicate which red number the

red arrow points to 115

83

366 Evaluation of Accommodation Functions

a Accommodation Posture The assessment of accommodation accuracy was performed at

the subjects habitual near distance using the monocular estimation method (MEM) The

MEM technique uses a specially made target with pictures or words suitable to the childs

reading level The card was attached to the retinoscope The procedure was performed

over subjects distance prescription and at his normal reading distance with enough light

to provide a comfortable reading vision Retinoscopy was performed on axis (right eye

before left eye) as the child reads the words or describes the picture on the card aloud As

the retinoscopic reflexes were observed an estimate of the magnitude of the motion was

made and trial lenses were then interposed briefly (approximately 05 seconds) to avoid

changing the childs accommodation status until no motion was observed The lens

power at which the motion was neutralized was taken as the lag or lead of

accommodation It is a lag of accommodation when the reflex is neutralized with

positive lenses while it is called lead of accommodation if neutralized with negative

lenses 112113116117

b Accommodation Facility this assesses the rate at which accommodation can be

stimulated and relaxed repeatedly during a specific time period It relates to the

individualrsquos ability to shift focus quickly and efficiently for varying distance The

procedure was performed with the best subjective correction in place monocularly and

binocularly but only the result for the binocular assessment was utilized for the data

analysis According to Wick and Hall 120 binocular accommodative facility testing may

not be a true accommodative facility measure such as monocular facility but clinically

binocular testing may be more useful because it gives a ldquoreal liferdquo assessment of

accommodation and binocular interactions of accommodation and vergence

The target used was letters on a 69 range on a near point card and at the subjectrsquos

habitual reading distance A lens flipper of 2 diopter lenses was used Letters and

pictures used were appropriate to the childs ability The subject was informed that he

would be shown two different lenses and that one pair causes the system to work while

84

the other pair relaxes it and was instructed to look at the materials as the lenses were

flipped Each time the print become clear and single he was to immediately report

clear The lenses were flipped and child should report clear when the material gets

clear To ensure that subject understood the instructions trial readings were first taken

The number of cycles completed per minute was recorded One cycle meaning clearing

both the plus and minus lens sides The test was done binocularly112-113118-120 At near

with no suppression control

c Amplitude of Accommodation The amplitude of accommodation was assessed by

Donderrsquos push ndash up method using a Royal Airforce (RAF) near point rule a rod with a

movable target and metrics as well as dioptric marking This procedure was performed

monocular and binocularly The subject was seated comfortably and instructed to read a

line of letters on the card that corresponded to a visual acuity of 69 and told to keep the

letters clear The target was slowly moved towards the child along the rule until the child

reported first sustained blur at which point the dioptric result was read off the Royal air

force rule Three readings were performed and an average taken This test was

conducted monocularly for each eye and then binocularly

As reported by Wick and Hall 120 the minimum amplitude of accommodation expected is

based on the subjects age and can be estimated from the Hofsetterrsquos formula

Minimum =15 - 025 (age)

Expected =185 - 03 (age)

Maximum = 25 - 04 (age)

The amplitude of accommodation was generally considered clinically important only

when it falls below the expected age minimum 120

d Relative Accommodation (RA) Relative accommodation tests are plus-to-blur (negative

relative accommodation) (NRA) and the minus-to-blur (positive relative accommodation)

(PRA) The relative accommodation tests assess patientsrsquo ability to increase and decrease

accommodation under binocular conditions when the total convergence demand is constant 106 It is also an indirect assessment of the vergence system since the vergence demand

85

remains constant while accommodative demand varies 121122

Relative accommodation was measured using the plus and minus lenses binocularly at

near with distance subjective results in place 122 In this procedure the child viewed a

reading target size letter size or picture of 66 size at the reading distance with the best

subjective correction in place Plus or minus lense powers was added in 025 diopter

steps in approximately 2 seconds interval121 to assess negative and positive

accommodation respectively This test was performed binocularly and the reading was

taken when the subject reported a sustained blur The relative accommodation is difficult

to assess with trial lenses121 The trial lenses were used to assess relative accommodation

in the present study The use of the phoropter was not possible considering the study

setting

367 Fusional Reserves

The term fusional reserve is used synonymously with vergence reserves and prism vergence

vergence amplitude 121123 or fusional amplitudes 111 Vergence ability is measured using

prisms placed in front of each eye The amount of prism is increased gradually to measure the

amount of fusional reserve the individual has in reserve to compensate for a phoria112 124 The

amount of base out prism required to produce diplopia is called positive fusional reserves or

positive fusional vergence while the amount of base in prism required to produce diplopia is

called the negative fusional reserves or negative fusional vergence 115

Positive fusional vergence (convergence) and negative fusional vergence (divergence) were

assessed using a prism bar in steps without suppression control although it has been reported

by Wesson et al 125 that when suppression is controlled the average vergence values will be

lower because the test is stopped when the suppression is detected that is if suppression is not

monitored the break is not detected until the stimulus is outside the suppression zone and a

higher vergence value may be obtained However

Scheiman et al 126 argued that the significance of such suppression in binocular individuals is

unknown

86

Assessing fusional reserves in children with loose prism bars may be a better technique

because it offers the possibility of viewing the eye movement objectively Secondly it

removes the restriction imposed by testing children behind the phoropter Finally the test

itself nearly duplicates the movement of the eyes under normal conditions allowing

convergence or divergence in discrete steps rather than as a smooth-pursuit type vergence

movement 125

The test was performed with the target at six meters (test target 69 letter line) and at 40 cm

(N5 text) The prism was placed in front of the right eye first Base-in was routinely assessed

first the rational for this sequence of testing being that the convergence response stimulated

during the base-out (or convergence) measurements may produce vergence adaptation ( a

fusional after-effect) which may temporarily bias the subsequent base in values in the base-out

direction 127 Typically there is no blur point for base -in vergence testing at distance This is

because at distance accommodation is already at a minimum and cannot relax beyond this

point 115

Base out prisms were used to measure positive fusional vergence while base in prisms were

used to measure negative vergence The subject was seated comfortably and instructed as

follows ldquoI am measuring the ability of your ayes to converge and diverge Watch the row of

letters Tell me when they first get blurry if they do Also tell me when they first become

two rows of letters and when they become one againPrime The prism was introduced over the

right eye lsquoDoublersquo was first demonstrated to the child by placing a 20 diopter prism over one

eye If blur was reported the prism power was increased until a break was reported The

prism power was then reduced until the subject reported that the rows of letters were single

The technique was then repeated for base out prism and then the test was repeated at near The

break and recovery points were determined subjectively from the childrsquos report of blur break

and recovery and objectively by observing the subjects eye movements The objective

findings were used for analysis

87

368 Ocular Health

The only test performed to evaluate ocular health was direct ophthalmoscopy

369 Vergence Facility

It was impossible to perform the vergence facility testing as the children could not fuse the

base in prisms So vergence facility data was not available for analysis The recovery point

of the base in reserves may provide useful information on the vergence facility According to

Bishop 111 ldquoblur and break points tend to reflect the quantity of fusion whereas the recovery

point indicates the quality of fusion that is the ease of change of fusional demands (facility)

and the ability to maintain fusion (stamina)

37 Statistical Analysis

The data entry was done by a staff optometrist who is skilled in statistical analysis The data

analysis was undertaken by the International Center for Eye Care Education statistician Data

was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 For comparison all data from both groups was subjected to a two-sample t-test (2-

tailed) (Table 32) Participants who did not meet the pass criteria for any of the test variables

were referred to their optometrist for further evaluation

Table 32 The Diagnostic Criteria for Each Test Variable

Variable Diagnostic Criteria

Visual Acuity 128-131 69

88

Refractive Errors 128-131

Hyperopia ge +075

Myopia ge -050

Astigmatism ge -075

Anisometropia ge 075 between the two eyes

Emmetropia lt - 050DS lt +100DS lt -075DCyl

Near Point of Convergence 132-134 ge 10cm

Accuracy of Accommodation 116117119120

Lag ge +075 Lead any minus finding

Accommodative Infacility 118119 gt 7cycles per minute in response to +2 -200 flipper testing

binocularly at near

Amplitude of Accommodation106119120

Binocular accommodative amplitude ge 2D below the expected

value for the patients age for minimum amplitude (using

Hofsetterrsquos formula) (15-14age)

Positive Relative Accommodation 106121122124

gt - 237DS

Negative Relative Accommodation 106121122124

gt + 2 DS

Phoria 106128

Distance

Near

gt 6 prism diopters exophoria or 4 prism diopters esophoria

gt 6 prism diopters exophoria or 4 prism diopters esophoria

Strabismus 131135 Manifest or intermittent manifest deviation of gt 2 prism

dioptres 135 or asymmetry in Purkinje reflex or compensating

fixation a misalignment of ge 25 degrees 131

Fusional Reserves LimitsDistance 119

Near 106121126

Base in Blur X Break 6 Recovery 4

Base out Blur 10 Break 16 Recovery 10

Base in Blur 14 4 Break12 5 Recovery 7 4

Base out Blur 22 8 Break 23 8 Recovery 16 6

89

38 SUMMARY

The study consisted of two groups of 31 participants from two schools in Durban The study

group was children with learning difficulties from which the dyslexic children were selected

and the control group was selected from a mainstream school The mean age for the dyslexic

participants were 13 years while the mean age of the children from the control group was 11

years and 9 months A convenience sampling method was adopted to select the study

participants due to the limited number of subjects to select from

The vision functions investigated may be divided into three broad areas of visual acuity and

refraction binocular vision and ocular pathology which corresponds with areas of vision

function investigated in the literature review Standard optometric testing procedures were

used to assess the vision variables

Visual acuity was assessed using the LogMAR chart at both distance and near Refractive

error was determined objectively using a streak retinoscope (without cycloplegia) while the

child fixated a 660 optotype at 6 meters with a fogging lens to control accommodation

The near point of convergence was measured using the Royal Air force rule (RAF) Ocular

alignment was assessed using the cover test at six meters and 40cm for distance and near

respectively For distance cover test the test target was a letter from the line above the

subjectrsquos best visual acuity of the worse eye The coveruncover (unilateral) cover test was

performed to determine the presence of a phoria or tropia The alternating cover test was

performed to assess the direction and magnitude of the phoria or tropia The test conditions

were the same as the unilateral cover test The Maddox Wing was also used to assess near

phoria under normal room illumination Strabismus was assessed using the Hirschberg test

with a penlight held about 50cm from the childs face in the mid plane and was encouraged to

fixate the light with both eyes open The location of the corneal reflection on each cornea

relative to the centre of the pupil was then noted

The different aspects of accommodation functions were also measured The assessment of

accommodation accuracy was performed at the subjects habitual near distance using the

90

monocular estimation method (MEM) The accommodation facility was assessed using a 2

diopter lens flipper with the best subjective correction in place The target used was letters on

a 69 range on a near point card and at the subjectrsquos habitual reading distance The amplitude

of accommodation was assessed by Donderrsquos push ndash up method using a Royal Airforce (RAF)

near point rule This procedure was performed monocular and binocularly with a 69 visual

acuity as the test target The relative accommodation was measured using the plus and minus

lenses The test target is a reading letter target size or picture of 66 size at the reading distance

with best the subjective correction in place This test was performed binocularly and the

reading was taken when the subject reported a sustained blur

The fusional reserves were assessed using prism bars in steps without suppression control

The test was performed at 6 meters (test target 69 letter line) and at 40 cm (N5 text) Base out

prisms were used to measure positive fusional vergence while base in prisms were used to

measure negative vergence Ocular health evaluation was assessed using the direct

ophthalmoscope

Data was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 To compare the data from both groups all data was subjected to a two sample t-test

(2-tailed) The methods used in this study were the same as those used for similar tests in the

international literature reviewed in Chapter Two and the sample size was larger than most of

the studies presented

Chapter Four will outline the study results with the comparative and descriptive statistical

analysis

91

CHAPTER FOUR RESULTS

41 Introduction

This chapter presents the study findings and the analysis of results obtained The data for the

prevalence of visual acuity refractive errors near point of convergence accommodation

functions and heterophoria are presented in histogram while the descriptive statistics are

presented in tables

42 Prevalence of Vision Defects

The prevalence is an indication of the subjects that have either achieved or did not achieve the

pass fail criteria and is not a normal distribution of a particular condition The prevalence rate

of the vision functions is expressed in percentages The other data is expressed as mean (M)

and standard deviation (SD) and calculated for descriptive purposes at 95 confidence

interval (CI)

If an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants would be expected

421 Visual Acuity (VA)

i Dyslexic Group In total 32 of subjects in the dyslexic group had visual acuity

less than 69 while 68 had visual acuity of 66 and 65 The distribution of those

subjects with VA le 69 is as follows 65 (2 subjects) had VAs of 69 65 (2

subjects) had VA of 612 65 (2 subjects) had VA of 615 3 (1 subject) had

VA of 630 3 (1 subject) each had VA of 638 648 660 respectively (Fig

41)

ii Control Group In total 32 of subjects in the control group had visual acuity

92

less than 69 while 68 had visual acuity of 66 The distribution is as follows 3

(1 subject) each had VA of 69 612 615 624 while 65 (2 subjects) had VA

of 630 638 and 648 respectively (Fig 41)

Fig 41 Prevalence of Visual Acuity

The mean visual acuity for the right eye for the dyslexic group was 017 031 and 000

024 for the right eye for the control group There was no statistically significant difference

between the two groups (p = 029) (Table 41) The mean visual acuity for the left eye for the

dyslexic group was 020 033 and 000 024 for the control group There was no

statistically significant difference between the groups (p = 023) (Table 41) LogMAR acuity

of 020 is equivalent to 69 in Snellenrsquos notation (Appendix E Visual acuity conversion table)

Table 41 Descriptive Statistics for Visual Acuity Variable Group N ( 95 CI) Minimum Maximum p

Mean (LogMAR)

SD

VAcuityRE

LE

DyslexicControl

DyslexicControl

3131

3131

017000

020000

031024

033024

000000

000000

010090

100090

029

023

422 Refractive Error

i Dyslexic Group Twenty three percent (23) of the dyslexic subjects had

refractive errors while seventy seven percent (77) were emmetropic (defined as

93

retinosocpic findings of less than 05 myopia less than +075 hyperopia or less than

075 astigmatism) (Fig 42) The distribution is as follows 65 (2 subjects) had

myopia 65 (2 subjects) had hyperopia (both latent) As cycloplegia was not

used in assessing refractive error an estimate of the diagnosis of latent hyperopia

was made based on a high difference between the retinoscopic findings and the

subjective refraction results In the event that the increased retinoscopic finding

(that is more plus) did not blur the subjectsrsquo vision latent hyperopia was assumed

About 10 (3 subjects) had hyperopic astigmatism 65 (2 subjects) had

amblyopia (defined as visual acuity in either eye of 2040 or a two-line difference

in acuity between the two eyes with no improvement with pinhole) 141 and 65 (2

subjects) had anisometropia (defined as a difference of ge 075 in sphere or cylinder

between the two eyes) (Fig 43)

ii Control Group Twenty two and a half percent (225) of the participants from

the control group had refractive errors classified as follows 65 (2 subjects)

had myopia 3 (1 subject) had hyperopia and 13 (4 subjects) had astigmatism

and 65 (2 subjects) had anisometropia No participant had amblyopia

Fig 42 Prevalence of Refractive Errors (total

01020304050607080

Myopia HyperopiaAstimatism (Hyperopic)Anisometropia Amblyopia Emmetropia

Prevalence

Dyslexic GroupControl Group

0

5

10

15

20

25

Prevalence

Refractive Errors (total)

Dyslexic GroupControl Group

94

Fig 43 Prevalence of the Types of Refractive Errors

Refraction data for two children from the dyslexic group who had cataracts could not be

obtained due to poor reflexes and were excluded from the analysis

The mean spherical equivalent (SE) refraction for the right eye for the dyslexic group was 086

098 and 070 103 for the control group was There was no statistically significant

differences between the two groups (p=066) The mean spherical equivalent refraction of the

left eye for the dyslexic group was 057 101 and 049 109 for the control group There

was no statistically significant difference between the two groups (p= 092) (Table 42)

Table 42 Descriptive Statistics for Refractive Error Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

RE (SE)

LE (SE)

DyslexicControl

DyslexicControl

2931

2931

086070

057049

098103

101109

-125-350

-150-350

500350

400350

066

092

423 Near Point of Convergence

i Dyslexic Group Thirty three percent (33 10 subjects) had NPC break points

(objective) of greater than or equal to 10cm while about sixty seven percent 67

(20 subjects) had NPCs of less than 10cm (Fig 44) One child complained of being

tired and did not participate in this procedure and was therefore excluded

ii Control Group Forty eight percent (48 15 subjects) had NPC break point

(objective) of greater than or equal to 10cm Fifty two percent (16 subjects) had

95

NPC less than 10cm (Fig 44) Data on NPC for one child from the dyslexic group

was lost

The mean NPC break was 890 503 for the dyslexic group and 1260 870 for the control

There was a statistically significant difference between the two groups (p = 0049) The mean

NPC recovery was 1400 588 for the dyslexics and 2200 820 for the control group

There was a statistically significant difference between the two groups (p = 006) (Table 43)

Table 43 Descriptive Statistics for NPC Break and Recovery Points Variable Group N (95 CI) Minimum Maximum P

Mean SD

NPC Break

NPC Recovery

DyslexicControl

DyslexicControl

3030

3030

8901260

14002200

503870

588820

500400

600800

26003400

28003800

049

006

424 Ocular Health

Six and a half percent (2 subjects) in the dyslexic group had cataracts (unclassified) on both

eyes No pathology was detected in the control group

425 Heterophoria

i Dyslexic Group No subject in the dyslexic group manifested with a phoria at

96

distance

At near 3 (1 subject) had an esophoria greater than or equal to 4 prism diopters

3 (1 subject) had an esophoria of 3 prism diopters 65 (2 subjects) had

exophorias of greater than or equal to 4 prism diopters 3 (1 subject) had an

exophoria greater than or equal to 8 prism dioptres and 65 (2 subjects) had

exophorias of 6 prism diopters (Fig 45)

ii Control Group At both distance and near no subject in the control group had

phoria of greater than 2 prism diopters

Fig 45 Prevalence of Phoria at Near (40cm) with Maddox Wing

Four children from each group could not complete the test as they left to attend lessons The

mean exophoria for the dyslexic group was 163 261 and 180 042 for the control group

There was no statistically significant difference between the two groups (p = 059) The

mean esophoria for the dyslexic group was 350 070 and 2 000 for the control group

There was no statistically significant difference between the two groups (p= 046) (Table 44)

No tropias were observed

Table 44 Descriptive Statistics for HeterophoriaVariable Group N ( 95 CI) Min Maxi P

97

Mean SD

At NearExophoria

Esophoria

DyslexicControl

DyslexicControl

2727

2727

163180

350200

261042

070000

000100

300200

1000200

400200

059

0469

426 Accommodation Functions

a Accommodation Posture As shown in Figure 46 the distribution of accommodation

lag is as follows

i Dyslexic Group

Thirty nine percent (11 subjects) had a lag of accommodation (defined as + 075 or

greater) three and a half percent (1 subject) had lead of accommodation (defined as

- 025 or greater) while fifty seven percent had normal accommodation posture

ii Control Group

Forty two percent (13 subjects) had lag of accommodation About eleven percent

(11) (3 subjects) had lead of accommodation while about thirty nine percent had

normal accommodation posture

Three children from the dyslexic group could not continue with this test as they had to

leave to attend class activities while three children from the control group sought

permission to be out of the testing room at that point These children were excluded

from the analysis for binocular accommodation lag

98

Fig 46 Prevalence of Accommodation Lag ( ge+075)

The mean accommodation lag for the right eye was 091 038 for the dyslexic group

and 092 057 for the right eye for the control group There was no statistically

significant differences between the two groups (p =083) The mean for the left eye for

the dyslexic group was 085 036 and 091 048 for the control group There was no

statistically significant difference between the two groups (p = 061) (Table 45)

Table 45 Descriptive Statistics for Accommodative PostureVariable Group N (95 CI) Minimum Maximum P

Mean SD

Accomm lag

RE

LE

DyslexicControl

Dyslexic Control

2828

2828

091092

085091

038057

036048

000-050

-050-050

200200

125200

083

061

b Accommodative Facility As shown in Figure 47 the distribution of accommodation

facility was as follows

i Dyslexic Group

Forty six percent (46 13 subjects) had normal accommodative facility (defined as

0

10

20

30

40

50

Prevalence

Bi no cu lar A ccom m od ation Lag

D y s lex ic G r o u p

C on t ro l G ro u p

99

greater than 7cpm) binocularly at near while (54) (15 subjects) had

accommodation facility less than or equal to 7cpm (Fig 47)

ii Control Group

Sixty seven percent (67) (18 subjects) had normal accommodative facility

(defined as greater than 7cpm) while thirty three percent (33) (9 subjects) had

accommodation facility less than 7cpm

Three children from the dyslexic group and four from the control did not complete the test

as they indicated that they were tired The mean binocular accommodation facility was

686 cpm 274 for the dyslexic group and 885 369 for the control group There was a

statistically significant difference between the two groups (p=0027) (Table 46)

Table 46 Descriptive Statistics for Binocular Accommodative FacilityVariable Group N ( 95 CI ) Minimum Maximum p

Mean SDAccommodativeFacility at near

DyslexicBE

ControlBE

28

27

686

885

274

369

2

2

12

21

002

7

c Amplitude of Accommodation Monocular amplitude of accommodation in both eyes

and for both groups did not differ significantly (not more than 050D) except for one

subject who had a difference of 4 diopters between the two eyes The amplitude of

100

accommodation for two subjects who had latent hyperopia in the dyslexic group was

relatively low (600DS and 800DS) as compared to age minimum amplitude of

accommodation of 1175DS The amplitude of accommodation for two participants who

had cataracts were excluded from the analysis

Only the data for the monocular amplitude was analyzed When accommodation

amplitude is assessed monocularly it measures the response for each eye individually

and is particularly important to determine whether a patient has accommodative

insufficiency 133 The monocular amplitude of accommodation has been used to assess

amplitude of accommodation function in several studies 86 88100102-103 The mean for the

right eye for the dyslexic group was 1198 234 and 1287 108 for the control group

There was no statistically significant difference between the two groups (p =007) (Table

47) The mean for the left eye for the dyslexic group was 1214 214 and 1287 116

for the control group There was no statistically significant difference between the two

groups (p =022) (Table 47)

Table 47 Descriptive Statistics for Amplitude of Accommodation Variable Eye Group N ( 95 CI ) Minimum Maximum p

Mean SDAmplit of Acomm

RE

LE

DyslexicControl

DyslexicControl

2931

2931

11981287

12141287

234108

215116

810

810

2015

2015

007

022

d Relative Accommodation (NRA PRA) As shown in Fig 48 the results for the relative

accommodation were as follows

For the PRA all subjects in both groups had PRAs of more than -200DS For the

negative relative accommodation (NRA) 89 of the dyslexic subjects had NRA greater

than +200DS while 96 of subjects from the control group had NRA greater than

+200DS

101

Fig 48 Prevalence of Relative Accommodation (NRA+2PRA-200)

Three children from the dyslexic group and five from the control group could not

continue with the test for the assessment of relative accommodation as they had to attend

important class activities

The mean PRA for the dyslexic group was -623 117 and -606 063 for the control

group There was no statistically significant difference between the two groups (p

=051) The mean NRA for the dyslexic group was 322 079 and 311 047 for the

control group There was no statistically significant difference between the two groups

(p =068)(Table 48) The findings for RA were higher than the published norm This

may be due to the process of addition of trial lenses

Table 48 Descriptive Statistics for Relative Accommodation Variable Group N ( 95 CI ) Minimum Maximum p

Mean SDPRA

N RA

DyslexicControl

DyslexicControl

2826

2826

-623-606

322311

117063

079047

-900-700

200200

-400-500

60045

051

068

427 Fusional Reserves

The children in both groups either could not report or understand blur so the result for break

102

and recovery was used in all analysis of vergence function

a Base-in Vergence Reserves at Distance two children from the dyslexic group could not

complete all aspects of the fusional reserves assessment because they were tired These

children were excluded in the analysis

The mean base-in to break for the dyslexic group was 1469 683 and 1600 350 for

the control group There was no statistically significant difference between the two

groups (p =046) The mean base in to recovery for the dyslexics was 1172 620 and

1280 317 for the control group There was no statistically significant difference

between the two groups (p =049) (Table 49)

Table 49 Descriptive Statistics for Base-in Vergence Reserves at distanceVariable Group N (95 CI) Minimum Maximum P

Mean SD

BI to Break (dist)

BI to Recovery (distance)

DyslexicControl

DyslexicsControl

2931

2931

14691600

11721280

683350

620317

410

28

4022

3520

046

049

b Base-in Vergence Reserve at Near The mean base in to break at near was 1185 514

for the dyslexics and 1283 313 for the control group There was no statistically

significant difference between the two groups (p=029) The mean base in to recovery was

872 478 for the dyslexics and 1032 335 for the control group There were no

statistically significant differences between the two groups (p=017) (Table 410)

Table 410 Descriptive Statistics for Base-in Vergence Reserves for NearVariable Group N (95 CI) Minimum Maximum P

Mean SDBI to Break (near)

BI to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

11851283

8721032

514313

478335

26

14

2518

2015

029

017

103

c Base-out (BO) Vergence Reserves at Distance Four children from the control

group could not give a report on the recovery point Two children from the dyslexic group

could not complete the test The mean base out to break at distance for the dyslexic group

was 2706 925 and 2416 975 for the control group There was no statistically

significant difference between the two groups (p=024) The mean BO to recovery for the

dyslexic group was 1876 796 and 17 693 for the control group There was no

statistically significant difference between the two groups (p= 040) (Table 411)

Table 411 Descriptive Statistics for Base-out Vergence Reserves at Distance Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

BO to Break(Distance)

BO to Recovery(dist)

DyslexicControl

DyslexicsControl

2931

2927

27062416

18761700

925975

796693

10001000

400600

0004000

35003500

024

040

d Base-out Vergence Reserves at Near The mean base-out to break at near was 2160

1162 for the dyslexic group and 2109 842 for the control group There was no

statistically significant difference between the two groups (p =084) The mean base-out

to recovery at near for the dyslexic group was 1335 745 and 1555 625 for the

control group There was no statistically significant difference between the two groups

(p =016) (Table 412) The comparative and descriptive statistsics for all variables is

shown in Table 413

Table 412 Descriptive statistics for base-out vergences at near Variable Group N (95 CI) Minimum Maximum P

Mean SD

BO to Break (near)

BO to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

21602109

13351555

1162842

745625

8001000

600800

4040

3530

084

016

104

Table 413 Comparative Descriptive Statistics for Dyslexic and Control groups

Variable Dyslexics( 95 CI)

Control( 95 CI)

N Mean SD N Mean SD PVisual Acuity

RE 31 017 031 31 000 024 029LE 31 020 033 31 000 024 023

Refractive ErrorRE 29 086 098 31 070 103 066LE 29 057 101 31 049 109 009

NPC Break 30 890 503 31 1260 870 049NPC Recovery 30 14 588 31 2200 820 006

Heterophoria (Near)Exo 27 163 261 27 180 042 059Eso 27 350 070 27 200 000 046

PRA 28 -623 117 26 -606 063 051NRA 28 322 079 26 311 047 068

Amp of AccommodationRE 29 1198 234 31 1287 108 007LE 29 1214 215 31 1287 116 022

Accom Facility (bin) 28 686 274 27 885 369 003

Accomm LagRE 28 091 038 31 092 057 083LE 28 085 036 31 091 048 061

BI Break (Dist) 29 1469 683 31 1600 350 046BI Recovery (Dist) 29 1172 620 31 1280 317 049

BI Break (Near) 29 1185 514 31 1283 313 029BI Recovery (Near) 29 872 478 31 1032 335 017

BO Break (Distance) 29 2706 925 31 2416 975 024BO Recovery (Distance) 29 1876 796 31 1700 693 040

BO Break (Near) 29 2160 1162 31 2109 842 084BO Recovery (Near) 29 1335 7 45 31 1555 625 016

105

Summary Statistical tests were performed on the data to ensure scientific validity and were presented for

comparative and descriptive purposes The prevalence of visual acuity was similar between the

dyslexic and control groups and there was no statistically significant difference between the two

groups (p = 029 right eye 023 left eye)

The prevalence of total refractive errors was similar between the two groups and there was no

statistically significant difference between the two groups (p = 066 right eye and 092 left

eye) Hyperopia was slightly more prevalent in the dyslexic group than in the control group

The prevalence of myopia was the same in both groups Astigmatism was more prevalent in

the control group than in the dyslexic group The prevalence of anisometropia was the same in

both groups Amblyopia was more prevalent in the dyslexic group compared to the control

group

The control group had a higher near point of convergence break and recovery points than the

dyslexic group The NPC showed a statistically significant difference between the two

groups NPC break (p =0049) and NPC recovery (p =0006) The prevalence of exophoria at

near was higher in the dyslexic than the control group There was no statistically significant

difference between the two groups (p= 059) The prevalence of esophoria in the dyslexic

group was higher than that found in the control group but there was no statistically significant

difference (p =046)

The prevalence of relative accommodation was similar in both groups and there was no

statistically significant difference between the groups The dyslexic group had a reduced

amplitude of accommodation compared to the control group and there was no statistically

significant difference between the two groups (p=007 right eye andp = 022 left eye) The

dyslexic subjects performed significantly worse than the control subjects in accommodative

facility function and there was a significant difference between the two groups (p =0027)

The dyslexic subjects had a lower accommodation lag than the subjects from the control

group There were no statistically significant differences between the two groups right eye (p

= 083) and left eye (p = 060)

At near the negative fusional vergence (base in vergences)) was reduced in the dyslexics

compared to the control group while the positive fusional vergence (base out vergences) was

similar for both groups There was no statistically significant difference between the groups

106

(p = 021 for BI break at near) and (p = 084 for BO break at near) Having analyzed the

results Chapter Five will discuss the findings in the context of the international studies

reviewed in the literature

107

CHAPTER FIVE DISCUSSION

51 Introduction

A review of the literature reveals that studies 2356878990 conducted on dyslexic children have

investigated the prevalence of vision defects in the dyslexic population and examined whether

such vision anomalies are correlated with dyslexia by comparing vision characteristics in

dyslexic subjects to normal readers

The aim of this study was to determine the prevalence of vision defects in a South African

population of the dyslexic school children and investigate the relationship between vision and

dyslexia by comparing the vision characteristics of the dyslexic to the control group of learners

from a mainstream school The proposed null hypothesis was that there was no statistically

significant difference between the means of the vision functions in the dyslexic population

compared to the control group A discussion of the results of this study is presented

52 Prevalence of Vision Defects

521 Visual Acuity

The 32 prevalence of visual acuity worse than 69 was the same for both groups A reduced

visual acuity could be due to ocular abnormality (such as cataract) or due to normal refractive

error differences in the population Visual acuity defects due to ocular diseases are usually not

common in paediatric populations142 but tend to be related to refractive error changes in the

population143 This relates more in the present study as the prevalence of defects of visual acuity

and the total refractive errors were similar Consequently in relating visual acuity to refractive

errors myopia astigmatism and hyperopia may reduce distance or near visual acuity

108

Visual acuity testing is widely performed in screening refraction and monitoring of disease

progression It is also used in both basic and clinical vision research as a way to characterize

participants visual resolving capacities144 Specific to the present study visual acuity testing

helps detect visual problems at the distances at which most school learning activities occur and

gives useful preliminary information on how good or poor a childrsquos vision is reduced distance or

near visual acuity gives an indication of poor vision that will necessitate further examination

However in assessing how visual acuity relates to reading performance it is important to note

that visual acuity as measured with the traditional letter identification only assesses the ability to

discern letters on the visual acuity charts and does not assess dynamic vision behaviour

binocular eye control or ocular stress and so may not necessarily mean normal vision

Furthermore normal visual acuity may not necessarily mean normal vision since some people

may have other vision defects such as color vision or reduced contrast or inability to track fast-

moving objects and still have normal visual acuity The reason visual acuity is very widely used

is that it is a test that corresponds very well with the normal daily activities a person can handle

and evaluates their impairment to do them 145

In relation to previous studies the visual acuity findings in this study are similar to reports by

Buzzelli 6 Goulandris et al 90 and Metsing and Ferreira 103 On the contrary Hoffman 83 found

a lower prevalence (1028) of visual acuity worse than 69 in a group of learning disabled

children In contrast to the present study however the mean age of subjects in Hoffmanrsquos study

was 8 years 6 months while the mean age in the present study was 13 for dyslexics and 11years 9

months for the control group Grnlund et al 140 documented that age must be taken into

consideration when describing and comparing VA in different populationsbecause VA develops

from birth to adolescence A lower prevalence (10) of visual acuity worse than 69 was also

reported by Sherman84 Latvala et al 3 reported a slightly worse distance acuity in the dyslexic

group than in the control group but noted no statistically significant difference Evans et al 5

found that dyslexic groups had a significantly worse visual acuity than the control group

(Binocular distance VA p =00164 and binocular near VA p =00018) Similarly Ygge et al2

reported that the subjects from the control group had a better visual acuity than the dyslexic

group at both distance and near and that the results showed statistically significant differences

(distance at p =003) (near at p=0005) Bucci et al 89 and Kapoula et al 87 reported the

109

prevalence of visual acuity of all participants in their study to be normal A study conducted by

Grisham et al 86 lacked a control group but it was reported that 568 of the participants had 66

acuity in both eyes (compared to 68 in dyslexic group in the present study) 172 had 69 or

worse (compared to a total of 315 who had visual acuity of 69 and worse in the present study)

The mean age of the participants in the study by Grisham et al 86 was 154 years while the mean

age of the participants in the dyslexic group in the present study was 13 years

The factors which may affect the outcome of visual acuity measurements that may lead to

variations in results reported by different authors include size position and distribution of retinal

mosaics (retinal photoreceptors) 143 optical aberration pupil size clarity of the optical media

age method and type of chart used contrast isolated or multiple letters state of accommodation

illumination as well as the criteria used to define visual acuity114 140 Psychological factors

affecting visual acuity testing results include blur interpretation fatigue and malingering 143

These factors may be difficult to control and could constitute confounding variables which may

lead to variations in visual acuity results reported by different authors

522 Refractive Error

The prevalence of uncorrected refractive error of 23 found in the dyslexic group was similar to

the 225 found in the control group and there was no statistically significant difference between

both groups (p = 066 right eye p = 092 left eye) This finding may mean that the dyslexic

subjects are not at more risk of a particular refractive anomaly compared to participants from the

control group as the dyslelxic gourp has similar distribution of refractive errors

Some studies 5838498103146 did not classify refractive error according to the types but presented

the results for the total refractive error On this basis the prevalence of total refractive error

(23 for dyslexic and 225 for the control group) in the present study is similar to reports by

other studies Hoffman83 reported a 215 prevalence of refractive error in a study of the

incidence of vision difficulties in learning disabled children Although there was a marked

difference in the sample size in both studies the similarities in the findings may be because as

with the present study Hoffmans study was based on a population of learners in a special school

110

referred for optometric care by educators psychologists and reading specialists

Similar to the present study Evans et al 5 and Ygge et al 2 reported that the prevalence of

refractive errors in the dyslexic and control groups was similar Metsing and Ferreira103 found

no statistically significant difference between the learning disabled and the mainstream

groups in their study Contrasting findings were reported by Sucher and Stewart 98 who found

the prevalence of refractive errors to be 94 for the control group although the 208 found in

their dyslexic group is similar to the 23 found in the present study Wesson 146 reported the

prevalence of refractive error in children with reading dysfunction to be 38 while the

prevalence of refractive errors was 16 in the study by Sherman 84 In the study by Alvarez and

Puell 88 the prevalence of refractive error was 424 in the dyslexic group while 415 of the

control group had refractive error When compared to the present study the higher prevalence of

refractive error in the dyslexic group can be accounted for by the relatively high prevalence of

hyperopia (287) and in the control group the higher prevalence of refractive error can be

accounted for by the high prevalence of myopia (194) It is unclear why the prevalence of

hyperopia was as high as no information was given on how refractive error was assessed

Statistically for the present study the mean refractive errors for the dyslexic group were right

eye 086 098 left eye 057 1013 and right eye 070 103 left eye 049 109 for the

control group There was no statistically significant difference (RE p =066 LE p = 092)

Evans et al 91 reported a mean spherical refractive error of 077DS (right and left eye similar) In

the study by Alvarez and Puell 88 the refractive error results (descriptive statistics) reported

were as follows for the poor readers mean spherical equivalent refractive errors for the right and

left eye were 020 06 and 020 06 respectively In the control group mean spherical

equivalent refractive errors for the right and left eye were - 020 08 and - 014 08

respectively The descriptive statistics on refractive errors was not reported in the studies 3 93

cited earlier which made it impossible to compare the statistical findings with the present study

In the present study about 68 of participants from both groups were emmetropic (defined as lt -

050DS +100DS and 075D cyl) while 655 of the dyslexic children and 645 of children

from the control group were emmetropic in the study by Alvarez and Puell 88 Only the study by

111

Alvarez and Puell 88 reported on emmetropia

Refractive errors are among the leading causes of visual impairment worldwide and are

responsible for high rates of visual impairment and blindness in certain areas School children

are considered a high risk group because uncorrected refractive errors can affect their learning

abilities as well as their physical and mental development147

Several factors may lead to variations in results found in different studies This include the type

of population studied (clinical or non-clinical) sampling method (convenience or cluster)

classification criteria examiner bias and more specifically the use of cycloplegia in the

assessment of refractive errors

The following studies2 3 8895 categorized refractive errors as hyperopia myopia astigmatism

anisometropia or amblyopia

a Hyperopia In the present study hyperopia appeared slightly more prevalent (65) among

the dyslexic group than in the control group (3) This result agrees with the report from

other studies 93 95 and Alvarez and Puell88 who reported a much higher prevalence (287)

for the dyslexic group compared to 161 of the control group Helveston85 found no

difference in the prevalence of hyperopia between the population of normal and poor

readers

Hyperopia is the refractive error that is consistently reported 3939598 to be associated with

reading difficulties so it was expected that one will find the higher prevalence of hyperopia

in the present study

In the present study two children had latent hyperopia (based on the assumption that

increased plus did not blur the distance vision) The full magnitude of the hyperopic

findings could not be estimated because cycloplegia was not used Therefore it is possible

that the prevalence of hyperopia may have been under-estimated Williams et al 148 studied

the ldquoprevalence and risk factors for common vision problems in childrenrdquo and concluded

112

that the 48 prevalence of hyperopia in their study was ldquoan underestimate as data was

collected without cycloplegiardquo Borghi and Rouse (cited by Scharre and Cree)149

determined that on an average 063D more plus was identified by cycloplegic refraction

In relating hyperopia to reading difficulties simple to moderate hyperopia may not cause

constant blur at a distance or near point but the extra accommodative effort produces

asthenopic symptoms of intermittent blur headache fatigue and loss of concentration and

inattention in some patients which may be mistaken for short attention span Uncorrected

hyperopia is associated with esophoria at near point which can stress the fusional vergence

systems that hold the eyes in correct alignment If the hyperopia and esophoria is excessive

an accommodative esotropia can result 81150 This may result in difficulty in reading

b Myopia The 65 prevalence of myopia was the same in both groups This result is similar

to an equal prevalance of myopia reported by Eames 95 (4 in both groups) Alvarez and

Puell 88 reported that 57 of dyslexics were myopic while 194 of the control group was

myopic The prevalence of myopia was higher in studies by Rosner and Rosner 93 (54

non- learning disabled and 19 learning disabled) and lower than the prevalence reported

by Grisham and Simons150 In contrast to the present study subjects from the study by

Rosner and Rosner 93 were patients of an eye clinic where the two main reasons for referral

were reduced visual acuity and school learning problems while subjects for the present

study were referred to the special school mainly due to low academic performance which

may not necessarily be vision- related The subjects were chosen for the study irrespective

of whether they complained of a visual impairment or not whereas in the study by Rosner

and Rosner 93 study subjects had been identified as having a visual impairment Therefore

the higher prevalence may have been from the selection protocol used by Rosner and

Rosner 93 in their study

The prevalence of myopia was not reported in several studies 3568498146 referenced but was

presented as ldquorefractive errorsrdquo (was not classified as myopia hyperopia or astigmatism)

The onset and progression of myopia may be influenced by factors such as environment

nutrition genetic predisposition premature and low birth weight the effect of close work

113

racial and cultural factors differences in pupil size and illumination143151 Variations in the

prevalence of myopia reported by different authors may be related to the variation in the

above-mentioned factors

c Astigmatism

The prevalence (13) of astigmatism in the control group was higher than in the dyslexic

group (10) and there was no statistically significant difference between the two groups

Similarly Alvarez and Puell 88 found a higher prevalence of astigmatism in the control

group compared to the dyslexic group astigmatism was detected in 161 from the control

group compared to 8 and 92 right and left eyes respectively from the dyslexic group

In contrast Latvala et al 3 reported that 36 in the dyslexic group had astigmatism and

none from the control group had astigmatism Ygge et al 2 reported that the prevalence of

astigmatism was higher in the dyslexic group (28 left eye 25 ) compared to the control

groups (right eye 183 left eye 243) although there was no statistically significant

difference between the groups (p=025)

Rosner and Rosner93 reported that 30 of non-learning disabled and 27 of learning

disabled participants had astigmatism which differed markedly from the findings in the

present study despite the similarities in the study population Eames95 found an equal

prevalence of astigmatism (7) between the two groups studied

Astigmatism affects vision in different ways Clinically astigmatism over 150 diopters can

often cause severe eye strain and interfere with reading and even lesser degrees of

astigmatism can be symptomatic in some patients 81

Naidoo et al 157 studied refractive error and visual impairment in African school children in

South Africa Although the study was conducted specifically on mainstream school

children it relates to the present study as it provided useful information on the visual

characteristics of school children in South Africa and forms a basis for comparison since all

the studies reviewed in the present study were conducted on Caucasian population except

114

for the study by Metsing and Ferreira 103 which did not provide much information on

refractive errors in the their study of learning disabled children in Johannesburg South

Africa

Naidoo et al 157 reported a prevalence of hyperopia of 18 myopia of 29 astigmatism

of 67 and 68 right eye and left eye respectively In comparison the prevalence of

refractive error for the control group in the present study was hyperopia 3 myopia 65

and astigmatism 13 The refractive error trend (astigmatism being higher in prevalence

than myopia and hyperopia) was similar between the present study and the study by Naidoo

et al 157 despite the difference in prevalence reported Given that ethnic origins culture

socio-economic class are comparable between both studies the difference in prevalence

may (partly) have been due to different sampling methods (cluster versus convenience) and

the use of cycloplegia in their study The use of cycloplegia has been reported to yield

spherical aberrations and unpredictable errors due to associated mydriasis 110

It has also been documented 158 that the use of cycloplegia yields more positive retinoscopic

findings although this did not seem to apply in this comparison as the prevalence of

hyperopia in the present study was higher than that reported by Naidoo et al 157 A possible

explanation for the difference in prevalence may be related to the different criteria used to

define hyperopia Bennet et al 159 stated that the degree of ametropia ldquohellip considered to be

within normal limits in a particular study will drastically affect the prevalencerdquo Similar

opinion was noted by Gronlund et al 140 who stated that ldquothe choice of definition will of

course have a great impact on prevalence In their study on Swedish children aged 4-15

years Gronlund et al 140 defined astigmatism as ge 075 and stated that the prevalence of

astigmatism would have been 21 (instead of 32) had they defined astigmatism as ge

100 Furthermore Grisham et al 86 in their ldquoStudy of poor readers in high schoolrdquo noted

that helliprdquoif one selected less stringent criteria then the proportion of students with any given

dysfunction would decreaserdquo In the present study the criteria used to classify hyperopia

were ge +100D while Naidoo et al 157 used ge +200DS cut- off

115

d Anisometropia Anisometropia may be defined as a difference of 075 (sphere or cyl) or

more between the two eyes and this definition was the criteria used in this study 129

Anisometropia is of great clinical interest because of its intimate association with

strabismus and amblyopia141 152

The 65 prevalence of anisometropia was similar in both groups This is comparable to

results reported by Latvala et al 3 where the prevalence of anisometropia was 36 (2

subjects) for the dyslexic group and 6 (3 subjects) for the control group but contrasts with

findings by Eames 95 who reported the prevalence of anisometropia to be 13 for poor

readers and 6 for the control group while Ygge et al 2 found a higher prevalence of

anisometropia in the control group (158) than in the dyslexic group (94)

In anisometropia the difference in refraction as well as the refractive error causes the image

to be out of focus on one retina blunting the development of the visual pathway in the

affected eye 141 The fovea of an anisometropic eye receives images from the same visual

object however the images from the more myopic or hyperopic eye are out of focus 153 It

appears that anisometropia is a major cause of amblyopia for at least one third of all

amblyopias 131 and anisometropia is considered to be a causal factor in the pathogenesis of

amblyopia and strabismus It is also estimated that 6 to 38 of all cases of amblyopia are

caused by anisometropia without strabismus whereas in about 12 to 18 of the children

with strabismus this is accompanied by anisometropia 154

According to Grosvenor 114 uncorrected anisometropia even of a moderate amount may

induce eyestrain because it is impossible for the accommodation mechanism to maintain

clear images on the retina at the same time On the other hand large amounts of

anisometropia (2DS or more) may not cause symptoms as no effort is made to maintain

single binocular vision 114

Rutstein and Corliss155 investigated the relationship between anisometropia amblyopia and

binocularity and concluded that higher degrees of anisometropia generally cause deeper

116

amblyopia and poorer levels of binocularity for hyperopes but not for myopes and stated

that ldquoAlthough uncommon small amounts of anisometropia can cause moderate

amblyopiardquo However the results of the present study failed to support this claims as none

of the subjects had amblyopia resulting from anisometropia

In anisometropia the displacement or distortion of the image prevents the development of

fine visual perception in the occipital cortex and puts the child at risk for developing

amblyopia 141 Anisometropia causes poor reading skills probably through the mechanism

of poor sensory and motor fusion rather than reduced visual acuity It degrades binocular

coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 150

e Amblyopia Two subjects (65) in the dyslexic group had ambyopia due to cataracts and

none from the control group had amblyopia This result may be comparable to studies by

Latvala et al 3 who reported 36 (2 subjects) for the dyslexic group and that of Rosner and

Rosner 93 with a prevalence of 4 for the non-learning disabled group and 3 for the

learning disabled group Cataract is a major cause of amblyopia (deprivation ambyopia) by

causing an impediment to the visual axis 141 Other conditions that may lead to amblyopia

include strabismus (which causes the images on the retinas to be dissimilar) and uncorrected

refractive errors (especially as in anisometropia) Anisometropia is related to amblyopia

Amblyopia is a major public health problem It is the most common cause of monocular

vision loss in children and young adults Early recognition and prompt referral are crucial

especially during infancy and childhood to prevent permanent loss of

vision 141

523 Heterophoria

Heterophoria is the relative deviations of the visual axes when the eyes are dissociated 160 Some

degrees of heterophoria are considered normal for persons with normal binocular vision

Approximately 1-2 prism dioptres of esophoria or 1- 4 diopters of exophoria at distance should

117

be considered to be within normal limits161 At near 3-6 prism dioptres of exophoria is

considered normal (physiological exophoria) 161 A normal healthy eye is usually able to

overcome these small deviations and so it is described as being compensated161 However

esophoria is much less compatible with comfortable vision than is exophoria Any amount of

esophoria in a patient with visually related symptoms may be problematic 162

In the present study all subjects were orthophoric at distance This report corroborates reports by

other authors Mathebula et al 163 studied heterophoria in a South African population of school

children (mean age 10 16 years) and reported that the mean distance horizontal heterophoria

showed orthophoria Similarly Evans et al 100 reported that all subjects examined ldquowere

orthophoric at distance with cover testrdquo According to Dowley160 ldquothe high prevalence of

orthophoria is realrdquo and that ldquothe high prevalence of distance orthophoriahellip may be due to the

coordinating influences of the same mechanisms that subserves prism adaptationrdquo A similar

view was expressed by Walline et al 164 who studied the ldquodevelopment of phoria in childrenrdquo and

stated that ldquonearly all of our subjects were orthophoric at distancerdquo

The major findings in heterophoria was a 95 prevalence of exophoria at near which was more

in the dyslexic subjects than in the control group with no exophoria There was no statistically

significant difference between the two groups (p= 059) At near 3 (1 subject) of the dyslexic

group had an esophoria greater than or equal to four prism diopters An interpretation of the

above finding is that the higher prevalance of heterophoria may mean that the dyslexic subjects

may be more uncomfortable when doing near work than the nroaml readers A possible

explanation may be that according to von Noorden (cited by Kommerell et al)165 heterophoria

typically causes asthenopia Patients with asthenopic symptoms (feeling of heaviness dryness

and soreness of the eyes pain in and around the eye frontal and occipital headache and the eyes

are easily fatigued) often have an aversion to reading and studying Typically such complaints

tend to be less severe or to disappear when patients do not use their eyes in close work165 It has

been documented that decompensated heterphoria (heterophoria accompanied by symptoms) can

be due to poor fusional reserves problems with sensory fusion (such as in anisometropia) or may

be due to an unusually large phoria which may be due to anatomical reasons or uncorrected

hyperopia 161166 The presence of phoria could contribute to visual and attention abnormalities

118

noted among children with reading difficulties making near point visual activities more

strenuous

Nearpoint esophoria has been reported to be associated with childhood myopia progression 164167

In the present study the only subject that had four prism diopters esophoria at near in the

dyslexic group had +125 hyperopia while the subject that had 3 prism diopters esophoria had -

150 DS myopia However due to the small sample size in the present study it is difficult to

draw any conclusion on the association between myopia and esophoria

The findings on heterophoria as in other aspects of vision functions in the dyslexic populations

reviewed are mixed However similar to the present study Latvala et al 3 assessed near

heterophoria using the Maddox Wing and reported a higher prevalence of near exophoria in the

dyslexics than in the control group Similarly Kiely et al 12 documented that dyslexics tend to

be more exophoric than normal readers Evans et al 101 found a higher prevalence of lateral

phoria in learning disabled subjects compared to normal readers In contrast to the above

findings Sucher and Stewart 98 assessed phoria using the cover test (testing distance was not

indicated in the results) but found a slightly higher prevalence of phoria in the control group than

in the dyslexics Evans et al 100 assessed near horizontal phoria using the Maddox Wing as in

present study but found no difference in near horizontal phoria between the dyslexic and control

groups Bucci et al 89 reported that there was no difference in phoria results at both distance and

near between the dyslexic and control groups Metsing and Ferreira 103 found no statistically

significant difference between the learning disabled and the mainstream groups on

heterophoriaThe use of the cover tests for both distance and near to assess heterophoria may have allowed for

standardization in technique but may not have implied that more valid results will be obtained

The cover test and the Maddox Wing have been used simultaneously to assess heterophoria in

other studies3 100 The possible sources of variations in results reported by different investigators

include classification criteria and poor technique 168

524 Near Point of Convergence

119

In the present study the prevalence of a remote (greater than 10cm) near point of convergence

was higher in the control group with 48 than in the dyslexic group with a prevalence of 33

There was a statistically significant difference in the near point of convergence break (p=0049)

and recovery (p = 0006) between the two groups

It may be that children who do not have reading difficulties tend to read more often than children

who experience difficulty to read This is because with increasing ability to read there is likely

to be more demand on accommodation and convergence resulting in near point stress as well as

esophoria The normal readers use and exercise the accommodation and convergence system

more frequently According to Owens and Wolf-Kelly 169 near work has long been considered a

potential source of visual problems stating that in their study reading ordinary text at a near

distance for about one hour induced significant changes in the resting postures of both

accommodation and vergences The authors indicated that ldquonear work induces a recession of the

near point of accommodation or vergencesrdquo

The near point of convergence break is associated with changes in inter-pupillary distance with

age As inter-pupillary distance widens with physical growth the amount of convergence in

prism diopters increases for a given distance from the subject 132 Chen et al 170 studied near point

of convergence in children aged 1-17 years and reported that an increasing incidence of remote

near point of convergence with increasing age in their study might be due to the near work

demands of primary school which might create a different level of near point stress than the near

work conditions in pre- primary school years

Similar to the findings in the present study Evans et al 91 reported a statistically significant

relationship between NPC and reading retardation (p=0019) Contrasting findings were reported

by other authors Bedwell et al 171 studied visual and ocular anomalies in relation to reading

difficulties and reported that 466 of the group of ldquogood readersrdquo had abnormal near point of

convergence compared to 58 of ldquopoor readersrdquo Similarly Bucci et al 89 and Kapoula et al 87

reported that the near point of convergence was significantly more remote in dyslexics than the

control In the study by Latvala et al 3 1275 of the dyslexic group and only 2 from the

120

control group had a near point of convergence of worse than 8cm A statistically significant

difference between the two groups was reported (p=00385) Metsing and Ferreira 103 found

no statistically significant difference between the learning disabled and the mainstream

groups on the near point of convergence function In a study that lacked control subjects

Grisham et al 86 reported a lower prevalence (154) of NPC (9cm or further) in a group of high

school poor readers The authors measured NPC three times on each subject in other to detect

fatigue but stated that it should therefore be noted that fatigue may be a problem for poor

readers even if they are able to converge at 8 cm if they tire easily then the ability to sustain

convergence during longer periods of reading may suffer

In the present study the measurement of the near point of convergence was repeated three times

on each subject in order to detect visual fatigue that the children experience everyday This

approach was documented by other authors 868789 111-113 Some authors111-113 have recommended

the measurement of the near point of convergence several times in order to detect fatigue which

is often indicative of poor convergence fusion system which may affect distract a child from

reading 172

525 Accommodation Functions

a Accommodation Facility Accommodative facility assesses the rate at which

accommodation can be stimulated and inhibited repeatedly during a specific time period It

relates to the individualrsquos ability to shift focus quickly and efficiently for varying distances

and is extensively used in the reading process 6

Only the result for the binocular accommodative facility was recorded and was used for analysis

in the present study Siderov and Johnston 174 noted that monocular accommodation

measurements provides a direct evaluation of the dynamics of accommodative response while

binocular testing of accommodative facility provides similar information but also reflects the

interactive nature of the relationship between accommodation and vergences

For the dyslexic group 54 (15 subjects) had inefficient accommodation facility while only 33

121

(9 subjects) of the control group had inefficient accommodative facility Statistically the control

group had a better accommodative facility (885cpm 369) than the dyslexics (686cpm 274)

and there was a statistically significant difference between the two groups (p =0027) It has

been documented 100 that an abnormal accommodative facility could imply difficulty in changing

focus from far to near and subsequently lead to a lack of interest in learning An efficient facility

of accommodation is particularly important because a greater period in school involves changing

focus between the chalkboard to near task such as writing at a desk Low values of

accommodative facility have been associated with symptoms related to near point asthenopia134

The result of the present study corresponds with findings by Evans et al 100 who reported that

dyslexics appeared to be slower at a test of accommodative facility Similar results were reported

in other studies 128898 but contrasts with results reported by Buzzelli 6 Buzzelli 6 assessed

accommodation facility using a Bernell Vectogram that utilized a polarised target to control for

suppression Buzzelli6 assessed accommodation facility as follows patients were asked to clear

the number 9 on the Bernell Vectogram target at 40cm They alternately viewed the target

through a 2D lens The test was continued for 20 cycles (a cycle being clearing the plus and

minus lens each presented once) The total time in seconds was recorded for the right eye left

eye and both eyes The polarized target controlled for suppression The use of the vectogram

to assess accommodation facility by Buzzelli6 differs from the flipper technique that was used in

the present study Secondly there was no control for suppression in the present study The

difference in results found between the two studies may be due to the lack of control for

suppression in the pre sent study Siderov and Johnston174 stated that ldquoclinical measurements of

binocular accommodation facility could vary depending on whether or not suppression has been

monitoredrdquo

A total of 317 of the poor readers in the study by Grisham et al 86 had inadequate (9cm or less)

accommodative facility

Allison173 suggested that there are different norms submitted by different authors for

accommodative facility measurements and that apart from adhering strictly to recommended

norms for accommodative facility testing other factors to be considered when evaluating

accommodative facility include the difficulty and speed of responses to the plus and minus lenses

122

and whether the subjects fatigue easily 173 Kedzia et al (cited in Zadnik)175 documented that

the factors that affect the testing of accommodative facility include the size and position of the

text being read the complexity of the target reaction time of the child to call out the symbol and

the magnification or minification factors introduced by the lenses themselves The above

variables are factors to be considered in assessing the variations in the findings for

accommodative facility reported in this study as compared to other studies

b Amplitude of Accommodation Accommodation is the ability to adjust the focus of the

eye by changing the shape of the crystalline lens to attain maximum sharpness of the retinal

image of an object of regard The absolute magnitude of the accommodative response is

termed the accommodative amplitude 120

Statistically the dyslexic group had a slightly reduced monocular amplitude of

accommodation (mean 12D) compared to the control group (1287D) but there was no

statistically significant difference between the two groups (p=070) The slight difference

in the mean amplitude of accommodation between the two groups may have been due to the

two subjects from the dyslexic group who had latent hyperopia

Similar to the present study lvarez and Puell88 reported that monocular accommodative

amplitude was significantly lower in the group of poor readers Evans et a l91100 in two

seperate studies 91100 reported that amplitude of accommodation was reduced in the

populations studied Ygge et al 102 and Goulandris et al 90 found no statistically significant

difference between the dyslexic and control groups Grisham et al 86 found that 247 of

the children had amplitude of accommodation which the authors classified as ldquoweakrdquo or

ldquovery weakrdquo while 115 had accommodative amplitudes classified as ldquoborderlinerdquo In the

present study the mean amplitude of accommodation for each group was within normal age

norms according to the amplitude norms and Hofsteterrsquos formula 106161

In the study by Metsing and Ferreira 103 a higher percentage of low amplitude of

accommodation for the right and left eyes respectively (516 and 533) was found in the

mainstream group compared to 291 and 288 in the learning disabled group The

123

relationships between the mainstream group and reduced amplitudes of accommodation of

the right (p=004) and left eyes (p = 0001) were found to be statistically significant

(plt005) Furthermore the relationship between the mainstream group and reduced ampli-

tude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to be moderate and

that between the mainstream group and the right eyes (Cramerrsquos = 0286) to be low

When accommodation amplitude is assessed monocularly it measures the response for each

eye individually and the limiting factor is the magnitude of blur-driven accommodation

Monocular amplitude measures are particularly important to determine whether a patient

has accommodative insufficiency 107 The amplitude measured binocularly is usually

greater than the monocularly measured amplitude due to the influence of both the blur-

driven accommodative response and accommodative and vergence responses 120 175

Clinically the amplitude of accommodation is considered important mainly when it falls

below the expected age norm in which case the child may experience blur vision at near

Secondly a difference of up to 2D between the two eyes may also be considered clinically

significant 120

An inefficient accommodation function may lead to difficulties in learning as the focusing

system of the eyes play a major role in the learning process Children who suffer some

anomalies of accommodation are more prone to fatigue quickly and become inattentive than

those who have norrmal accommodation function 87 Symptoms of accommodation

insufficiency are specifically related to near vision work 176

Factors affecting the measurement of amplitude include differences when measurements

are taken monocularly as compared to when assessed binocularly the angle of gaze target

size age refractive error race and climate 175

c Accommodation Posture During near vision the eyes are not usually precisely focussed

on the object of regard but the accommodation lags a small amount behind the target If

the accommodation lag is small then the blur it causes is insignificant if high then it can

result in blurred print during reading 142

124

About 3928 (11 subjects) of dyslexic subjects had lag of accommodation compared to

4193 (13 subjects) of the control group The mean accommodation lag was similar in

both groups 092 for control and 088 for the dyslexic group There were no statistically

significant differences between the two groups right eye (p = 083) and left eye (p = 060)

Similarly Evans at al 100 reported that the mean accommodation lag did not show any

statistically significant difference between the dyslexic and control groups On the

contrary Metsing and Ferreira 103 found a high prevalence of lead of accommodation for

the mainstream group but a high prevalence of lag of accommodation in the learning

disabled group The relationships were found to be statistically significant (plt005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000) In another

study Evans et al 91 reported a mean accommodation lag of + 112DS

An individual with a lag of accommodation habitually under accommodates and may lead

to difficulty with reading An accommodative response that manifests as an excessive lag

of accommodation may indicate latent hyperopia esophoria or may be associated with

accommodative insufficiency or accommodative spasm poor negative vergences or when

patient is overminused 177 The prevalence of high lag may be related to the prevalence of

latent hyperopia and esophoria in the present study

C Relative Accommodation (PRA NRA) The relative accommodation tests assess the

patientsrsquo ability to increase and decrease accommodation under binocular conditions when

the total convergence demand is constant67 It is also an indirect assessment of the

vergence system since the vergence demand remains constant while accommodative

demand varies 121 The results for the relative accommodation for all subjects from both

dyslexic and control groups were unexpectedly high The norm for the mean NRA and PRA

were given as approximately +2 or - 2 diopters from two reports consulted 119 121 while the

relative accommodation results in this study ranged between 3 and - 6 diopters Similarly a

higher relative accommodative value was reported by Chen et aland Abidin139 in a study of

vergence problems in Malay school children The only available study accessed that

125

reported on relative accommodation was conducted by lvarezand Puell 88 who reported

that the negative and positive relative accommodation values were similar in both groups of

children Statistically the results were (NRA Control group 19 06 poor readers 19

06 PRA Control group 23 09 Poor readers 20 13) compared to the findings from

the present study (NRA Control group was 311 047 NRA dyslexic group 322 079)

(PRA Control ndash 606 063 PRA dyslexic was -623 117) The p values were not

indicated

The higher values found in the present study may be due to the process of addition of trial

lenses specifically The phoropter has been reported to be a better technique to assess

relative accommodation 119

Although the relative accommodation was not assessed by many authors different opinions

were expressed regarding the assessment of relative accommodation Latvala et al 3 noted

that positive and negative relative accommodation ranges are ldquodifficult to take and

unreliablerdquo According to Morgan 178 the PRA and NRA results are often unusually high

but recommended that such tests results can be used as a sort of indicative way and can only

be interpreted loosely and suggested that the use should be restricted to special

circumstances Garcia and Fransisco 122 reported that high NRA can be associated with

disorders such as accommodative insufficiency and convergence excess while high values

of PRA are related to anomalies in which accommodative excess appears but concluded that

PRA and NRA findings were used mainly as complementary diagnostics tests of some

disorders in literatures consulted

A high value of the negative relative accommodation could indicate that the children were

exerting maximum accommodation effort Because 250D of accommodation is exerted at

the 40cm working distance the maximum amount of accommodation that would be

expected to relax accommodation at 40cm would be 250D A value greater than 250D

would mean that accommodation may not have been fully relaxed during the subjective

refraction 114 Generally a high value of the NRA may also mean that the refraction may

126

have been under corrected for hyperopia or over corrected for myopia 122 Garciaand

Francisco122 reported the relationship between high NRA values and under corrected

hyperopia to be due to the etiology of hyperopia

According to Grosvenor 114 the amplitude of accommodation is a limiting factor for the

expected value of the PRA For example ldquoIf a patient has an amplitude of accommodation

of only 150D we would expect the minus-lens-to-blur test to be no greater than -150Dhelliprdquo

A high value of the positive relative accommodation is related to convergence insufficiency 122 Illumination and the range of fusional vergence also affect the relative accommodation

An excessive illumination will give an erroneously high finding due to the increased depth

of focus 100

Dysfunctions of accommodation can significantly interfere with the comfort clarity speed

and accuracy of reading as the child develops reading skills 51 Wick and Hall 120 suggested

that the results for the tests of accommodation are more meaningful when analyzed

together as the results of individual accommodation function may not give a true reflection

of the childrsquos accommodation dysfunction

526 Fusional Reserves

Fusional vergence amplitudes is a clinical measurement of a person`s fusional vergence ability It

reflects the ability of the oculomotor system to maintain sensory fusion in spite of varying

vergence requirements 179 The horizontal vergence reserves describe the amount by which the

eyes can be converged or diverged whilst the subject maintains fusion 100161 It is a measure of

how much fusional vergence is available in reserve that can be used to overcome a phoria The

amount of base out prism required to produce diplopia is called positive fusional reserves

(measures convergence) 100 The measurements of convergence fusion are also referred to as the

lsquoamplitude of fusional reserves 111 Clinically the fusional amplitude provides information about

a patientrsquos ability to maintain comfortable binocular vision 100112

The participants from both the dyslexic and control groups either could not report or understand

127

blur so the result for break and recovery was used in all analysis of vergence function Evans et

al 100 reported similar observations For the fusional reserves only the findings for the near

positive fusional vergence (convergence) are emphasized The positive fusional

reservesvergence (measures convergence ability) may be more important in assessing reading

dysfunction More so it has been suggested by other authors 112121180 that near measurements

are more relevant in assessing vision functions in dyslexic children

Based on Bishops 111 recommended norm of between 30 and 40 prism diopters for positive

fusional amplitude at near the prevalence of abnormal positive fusional vergence at near was

83 for the control and 74 for the dyslexics This appears unusually high Walters 168

explained that a possible reason for an unexpected high base out finding is that exophoric

conditions makes compensating for base out more difficult than base in testing It is unclear if

this is the case in the present study although the prevalence of exophoria at near in the present

study was 95 The compensation for a phoria is maintained by fusion reflex so knowing the

magnitude of the vergence amplitude that is needed to compensate a given phoria is very useful

clinically181

For the present study at near the negative fusional vergence (base in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base out

vergences) was similar for both groups Overall there was no statistically significant difference

between the groups (p = 021 for BI break at near) and (p = 084 for BO break at near) Evans et

al 100 reported that the dyslexics have reduced fusional reserves compared to the control group

Bucci et al 89 reported reduced divergence capabilities in dyslexics at near

When assessing fusional reserves the blur point is a function of the flexibility between vergence

and accommodation When little flexibility exists the addition of even low prism causes a

simultaneous accommodation shift with resultant blur Lack of low blur findings may indicate an

inadequate flexibility bond between accommodation and convergence The break point is an

indication of the quality of binocular function Break values will be adequate when fusion ability

is good while the recovery measure is a more subtle indicator of the quality of binocular function

Consequently reduced blur break or recovery findings indicate the presence of near point stress

128

182 Some subjects from both groups had break values higher than 40 prism diopters for base in

and base out reserves Wesson et al 125 in their study reported a similar observation on objective

testing of vergence ranges Such high break findings may be because suppression was not

controlled while assessing the fusional reserves in the present study When suppression is

controlled the average vergence values will be lower because the test is stopped when the

suppression is detected If suppression is not monitored the break is not detected until the

stimulus is outside the suppression zone and a higher vergence value is obtained 125 However it

is important to note that the significance of such suppression in binocular individuals is unknown 126 Again in special cases such as suppression large abnormal findings (compared to

normative ranges) may permit better functions in reading than a lower abnormal finding 183 For

example an individual who totally suppresses the vision of one eye is less apt to have difficulty

at reading than the individual who only partially suppresses the vision of one eye 183

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups The

mean fusional convergence capacities at distance were 1680 prism dopters for both the control

and the dyslexic group At near the corresponding figures were 2640 prism diopters and 2670

prism diopters respectively The mean fusional divergence capacity at distance was 650 prism

diopeters and 620 prism diopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 1050 and 1020 prism diopters respectively There was no

statistically significant difference between the groups

Latvala et al 3 reported a prevalence of fusional amplitude greater than or equal to 32 prism

diopters to be 61 for the control group and 75 for the dyslexic group using referral criteria

of 32 prism diopters at a distance of 33cm At near a testing distance of 40cm was used in this

study The difference in prevalence between the two studies might be from the different test

distances used Furthermore as detailed by Rouse et al 158 large intra subject intra-examiner

and inter-examiner variations in the assessment of fusional reserves have been reported

Vergence amplitudes have been reported to vary with alertness that is whether the subject is

tired or rested or under the influence of a toxic agent 184

129

Bedwell et al 171 reported that neither convergence nor divergence were significantly related to

reading difficultyrdquo Similarly Metsing and Ferreira103 found no statistically significant

difference between fusional vergences amplitude (Base-out base-in) at near No details were

provided

There are several limitations with the assessment of fusional reserves Sheedy (cited by Rouse et

al) 158 remarked that ldquo the general opinion is that when fusional vergence tests are repeated on

the same patient the second value found may be quite different from the first a difference of 10

PD from one fusional vergence amplitude measurement to another is not unusual unless rigorous

controls ar a typical difference of 3- 4 prism diopters but can measure differences as large as 12

prism diopter on follow up visit and that inter-examiner variation can be as large as 10 to 16

prism diopters The authors concluded that the positive fusional vergences difference could be

due to children having difficulty understanding the instructions or expected endpoints children

being slower responders or may be poorer observers The blur readings on the vergence reserves

for both distance and near were excluded in the analysis as it was difficult to get the children to

elicit a proper response It is possible that the children did not understand the instruction or the

concept of blur despite repeated explanations and trial readings This variable is often difficult

for young subjects to understand Similar problems were encountered by Evans et al 100 who

commented that several subjects were unable to appreciate a blurrdquo Scheiman et al 126 stated

that ldquowe were unable to consistently obtain a blur finding from our samplerdquo and that rdquo less

than13 of the subjects were able to report a blur we therefore only recorded the break and

recovery findingsrdquo

Wesson et al125 emphasized that the ldquomajor limitations of any vergence measurement

isrdquohellipnecessity to obtain a subjective response to blur break and recovery and that working with

children whose responses are not reliable is difficultrdquo It was suggested that a break and recovery

point could be determined by observing the subjects eye movements as prism power is increased

This technique was applied in this study However most children have difficulty maintaining

fixation long enough to measure vergence ranges

Vergence is influenced by several factors including awareness of the distance of the object

130

(proximal vergence)cross-linking with the accommodative system (acommodative vergence) and

the fine tuning of ocular alignment during the fusion of each monocular image into a single

percept (fusional vergence) 161 and other factors such as test target and lighting conditions 125

Generally in the assessment of a patientrsquos binocular status the fusional reserves may be more

useful when analyzed with the phoria measurements 115

The vergence system is closely related to the accommodative system and symptoms may at times

appear similar The symptoms associated with deficiencies with the vergence system include

letters or words appear to float or move around postural changes noted when working at a desk

difficulty aligning columns of numbers intermittent diplopia at either distance or near 185

Furthermore ldquodysfunctional vergence could cause an excess of eye movements especially of

small saccades The ability to continue the rapid decoding of the visual characters decreases as

more stress is placed on the vergence mechanism of the dyslexic during reading which makes

them tire more quickly than normal readersrdquo 6

A possible explanation for the relationship between vergence control and reading proposed by

Riddel et al (cited by Morad et al) 186 is that children with poor vergence control had impaired

accuracy of spatial localization that may impend their ability to accurately determine the position

of letters within words

527 Ocular Pathology

Two participants from the dyslexic group had cataracts Only the study by Hoffamn 83 reported

that one child had conjunctivitis

54 Summary

The results of this study do not vary significantly from the international studies in spite of the

differences in study designs tools and sample sizes Although studies have indicated differences

131

in the prevalance of open angle glaucoma and myopia between Caucasian and African

populations this study does not indicate a specific difference in the prevalance of vision variables

between Caucasian and African dyslexic school children However the lack of studies on

African children makes it difficult to reach any conclusions regarding structural differences that

may affect the presence of dyslexia Comparing the findings of this study with those done

internationally highlights the complexity of investigating the visual conditions that may be

associated with dyslexia and the need for rigorous and consistent testing methods in order to be

able to compare results

Having completed a discussion of the study findings in Chapter Five Chapter Six will present the

study conclusion and indicate its limitation as well as recommendations for future studies

132

CHAPTER SIX CONCLUSION

61 Introduction

Being able to read and write is an essential part of being able to interact with the world As

vision plays a major role in reading and the learning process a normally working visual system is

essential for efficient reading

With as many as 20 of Caucasian children being affected to a greater or lesser degree by

dyslexia 1631 there is a need to identify its cause and develop remedial actions to reduce its

effects As studies 4 59 have shown that intellectual ability is not the cause of dyslexia research

to enable people to reach their full potential in spite of their impaired reading ability requires a

multidisciplinary approach of which optometrists are part

This study aimed to determine the prevalence of vision conditions in a South African population

of African dyslexic children and to study the relationship between vision and dyslexia in an

African setting This was done by investigating visual acuity refraction and binocular functions

between two groups of 31 African school children one group attending a school for children with

learning difficulties and the other a mainstream school This enabled a study of possible vision

defects in African dyslexic children as well as a comparison between the two groups

The study targeted African school children its objectives being to

1 determine the distribution of visual acuity disorders among dyslexic children

2 determine the distribution of refractive errors among dyslexic children

3 determine the distribution of heterophorias among dyslexic children

4 determine the distribution of strabismus among dyslexic children

5 determine the distribution of accommodation disorders among dyslexic children

6 determine the distribution of vergence disorders among dyslexic children

7 determine the distribution of ocular pathology among dyslexic population

133

8 compare these findings to a similar group of non-dyslexic children

A review of the literature showed that most studies that investigated visual functions in dyslexic

school children were conducted on Caucasian population and that the findings of these studies

were inconclusive The broad areas of vision investigated in the previous studies 2-6 87-89 were

visual acuity and refraction binocular vision and ocular pathology

As studies conducted on the African population was lacking this study examined visual acuity

(using LogMar charts) refraction (static retinoscopy without cycloplegia) near point of

convergence (RAF rule) [accommodation functions 2 flipper lenses to assess accommodation

facility MEM retinoscopy for accommodation posture and pushndashup method to evaluate

amplitude of accommodation] ocular alignment (cover test and Maddox Wing) and strabismus

(Hirschberg test) and fusional reserves (using prism bars)

The study was conducted on a African population of 31 dyslexic school children selected from a

school for children with learning difficulties and 31 control participants from a mainstream

school in Durban Their ages ranged between 10-15 years The participants were selected using

the convenient sampling method as there were only a few participants classified as being

dyslexics All data collection procedures were conducted at the respective schools

The prevalence of vision conditions were presented in percentages () Data was analyzed using

the Statistical Package for Social Sciences (SPSS) and was expressed as mean (M) and standard

deviation (SD) and calculated for descriptive purposes at 95 confidence interval (CI) The level

of significance considered to support a hypothesis was taken as P lt 005 For comparison all

data from both groups was subjected to a two sample t test (2-tailed)

134

62 Summary of findingsIf an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants (control group) would be

expected

621 Visual Acuity

The prevalence of visual acuity worse than 69 (23 dyslexic and 225 control) is the same in

both the dyslexic and the control group and there was no statistically significant difference (p =029 023 right and left eye respectively This is similar to the findings by Buzzelli 6 and

Goulandris et al 90 which found no statistically significant difference between the two groups

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexia and the control group (p=029 for the right eye and 023 for the left eye)

622 Refractive Errors

The prevalence of refractive errors was similar in both groups There was no statistically

significant difference between the two groups (p=066 for the right eye and 092 for the left eye)

This is similar to the findings by Evans et al 2 which found no statistically significant difference

between the two groups (p=058)

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 066 for the right eye and 092 for the left eye)

a Hyperopia Hyperopia was more prevalent in the dyslexic group than in the control group

This is consistent with findings in other studies 88 93 95

b Myopia The prevalence of myopia was the same in both groups This is similar to the

findings by Eames 95 which found no difference in the prevalence of myopia between the

two groups

135

c Astigmatism Astigmatism was more prevalent in the control group than in the dyslexic

group This is similar to the findings by Alvarez et al 88 which found a higher prevalence of

astigmatism in the control group than in the dyslexic group

d Anisometropia The prevalence of anisometropia was the same in both groups (2 subjects

each) This is comparable to results reported by Latvala et al 3 where the prevalence of

anisometropia was 36 (2 subjects) for the dyslexic group and 6 (3 subjects) for the

control group

e Amblyopia Amblyopia was more prevalent in the dyslexic group compared to the control

group This is similar to the findings by Latvala et al 3 and Rosner and Rosner 93 which

found a higher prevalence of astigmatism in the dyslexic than in the control group

623 Near Point of Convergence

The prevalence of remote NPC (ge10cm) was higher in the control group than in the dyslexic

group and there was a statistically significant difference between the two groups (p= 0049) This

is similar to the findings by Kapoula et al 87 Latvala et al 3 and Evans et al 91 who found a

statistically significant difference between the dyslexic and the control groups on the NPC

functions

The null hypothesis was rejected as there was a statistically significant difference between the

dyslexic and control groups (p = 0049)

624 Heterophoria

The prevalence of exophoria at near was higher in the dyslexic group (95) than in the control

group with no exophoria There was no statistically significant difference between the two

groups (p= 059) The prevalence of esophoria in the dyslexic group was higher than that found

136

in the control group and there was no statistically significant difference (p =046) This is similar

to findings in other studies 3 12101163 which found no statistically significant difference between

the two groups

The null hypothesis was accepted as there was no statistically significant difference between the

dyslexic and control groups (p=059 for exophoria) and (p=046 for esophoria)

625 Accommodation Functions

a Amplitude of Accommodation The dyslexic group had a reduced monocular amplitude

of accommodation compared to the control group but there was no statistically significant

difference between the two groups (right eye p = 007 left eye p=022) This is similar

to the findings reported in other studies 3 87 91102 which found no statistically significant

difference between the two groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation amplitude (right eye

p=007 left eye p=022)

b Accommodation Facility The participants from the dyslexic group performed

significantly worse than the control group in accommodative facility function and there

was a significant difference between the two groups (p=0027) This result corroborates

findings reported in other studies 12 88 98100 which found a statistically significant

difference between the two groups

The null hypothesis was rejected as there was a statistically significant difference between

the dyslexic and control group in accommodation facility (p=0027)

c Accommodation Lag The prevalence of lag of accommodation was higher in the control

group compared to the dyslexic group and there was no statistically significant difference

There were no statistically significant difference between the two groups (right eye p=083

137

and left eye p= 060) This is similar to the findings by Evans et al 100 which found no

statistically significant difference between the two groups The mean accommodation lag in

the present study were 092D for control and 088D for the dyslexic which is comparable to

reports by Evans et al 91 who reported a mean accommodation lag of + 112D

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation posture

d Relative Accommodation (PRA NRA) The mean values for the relative

accommodation are similar in both groups and there was no statistically significant

difference between the groups (p=068 for NRA and p =051 for PRA) This is similar

to the findings reported Alvarez and Puell 88 which found no statistically significant

difference between the two groups in relative accommodation

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups in relative accommodation

626 Fusional Reserves

For the present study at near the negative fusional vergence (base-in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base-out

vergences) was similar for both groups There was no statistically significant difference between

the groups (p= 021 for BI break at near) and (p = 084 for BO break at near) This is similar to

the findings by Evans et al100 which found no statistically significant difference between the two

groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 021 for BI break at near) and (p = 084 for BO break at near)

627 Ocular Pathology

138

Two participants from the dyslexic group had cataracts

63 Implications of the Study Findings

a Causal Relationship This study presented the prevalence of some vision defects in a

South African African population of dyslexic school children as compared to a population

of non-dyslexic children and could not establish any causal relationship between vision and

dyslexia The vision parameters that showed that statistical significance may only be

reflective of factors associated with dyslexia but in a non-causal way This may mean that

the prevalence of the particular visual variable is much higher in the dyslexic population

than in the control group Vision is essential for reading but the vision anomalies that are

found to be associated with dyslexia also occur in children who do not have reading

difficulties It is therefore possible that other complex vision functions may be more related

to the etiology of dyslexia than peripheral visual factors such as NPC and accommodative

facility which showed statistically significant differences in the present study

b Risk Identification The findings of this study also suggest that dyslexic children are not

at more risk for the identified vision condition compared to non-dyslexic children

However even when vision defects may not be the cause of dyslexia the presence of

uncompensated vision defect in a dyslexic child may constitute difficulty while reading

and should be compensated for Improvement in vision will make reading easier for the

dyslexic child

64 The Study Strengths

The study methodology was based on reports found in other studies and consisted of the

following

a Classification The subjects selected for the dyslexic group represent a dyslexic population

that was classified according to the criteria used in the study by Latvala et al 3 and Evans et

139

al 100 The study was undertaken at a school for learning disabled learners and all students

were assessed and categorized as being dyslexic by the psychologists

b Study Examiner Examination of the subjects was conducted by the same optometrist who

had some experience in paediatric optometry This minimised the possibility of bias and

inter-examiner variability

c Examination Techniques The examination techniques used in this study were the same

as the techniques used in major referenced studies which were published in peer-reviewed

journals

d International Relevance It is the first study that has assessed broad vision functions in

a dyslexic population in an African setting

6 5 Limitations of the Study

The factors which may limit the generalization of findings of the study are as follows

a Inattentiveness This occurred in some children after numerous tests and the possibilities

of their lack of understanding of certain instructions However this problem was

minimized as trial readings were taken for some procedures and the participants were

instructed accordingly As stated in Section 34 testing was discontinued when the child

was tired This approach helped to minimize fatigue which could have negatively

affected the results In addition objective techniques that required minimal responses

from the participants were utilized in several procedures such as the cover test

retinoscopy and the monocular estimation method for evaluating accommodation posture

b Convenience Sampling This method of sampling was used given the limited number of

available subjects from the target population The available studies 235687-91102103 on

the subject of dyslexia learning disabilities and vision also used the convenience

sampling method

140

c Generalised Findings The generalization of the findings of this study is limited by the

fact that the refraction data was collected without the use of cycloplegia At the inception

of the data collection the diagnostic drug usage by optometrists was not approved in the

scope of practice in South Africa

d Sample Size The sample size was small as there was only one school for dyslexic

African children in Durban at the time of this study However review of the literature

also reveals that the sample sizes on studies conducted on dyslexia learning disabilities

and vision were typically small The targeted sample size of one hundred for this study

could not be met due to the limited number of subjects available The average sample size

of eight published studies 356 8790100 102 conducted on dyslexic children was 41

e Suppression Control The assessment of binocular accommodation facility and the

vergence reserves were performed without controlling for suppression

f Assessment of Relative Accommodation the use of a Phoropter would have yielded a

more conclusive result

g Eye Movements these were not assessed

Despite the acknowledged limitations inherent in this study the study provides useful

information and a research perspective on the prevalence of vision defects in a South African

population of dyslexic children which has not been conducted before

66 Recommendations

The following recommendations are made based on the findings of this study

661 Future Research

141

1 The same study with a similar protocol larger sample size with randomized sampling

2 Investigate the relative accommodation in in both groups using the phoropter

3 Assess the binocular functions with suppression control

4 Assess refractive error under cycloplgia

662 General Recommendations

1 All dyslexic children should have their eyes examined routinely to rule out

possibility of vision defects impacting on their reading performance

2 The Department of Education be reminded that visual defects can impact negatively on

the reading ability of learners and that teachers need to be more aware of pupils who may

present with signs of visual problems such as holding book too close or battling to see

the chalk board

3 The principals of the mainstream school should also be informed that vision defects were

detected in the non-dyslexic children

67 Conclusion

As a neurological condition which manifests primarily as a language-based disorder with

difficulty with words dyslexia requires a multi-disciplinary approach to its management with

an eye examination being needed to eliminate any possible effects that ocular conditions may

contribute to reading problems

The study provided the prevalence of disorders of visual acuity and refractive errors near point

of convergence accommodation dysfunction heterophoria strabismus and fusional reserves

among African dyslexic children of age range 10-15 years The only vision variable that was

more prevalent and that is significantly associated with dyslexia was the binocular

accommodation facility while only the near point of convergence (break and recovery) was

significantly more prevalent in the control than the dyslexic group The existence of these

statistically significant differences between the two groups may not imply clinical relevance

due to the small sample size The comparison of vision characteristics between the dyslexic

142

and control group indicates that the dyslexic children are not more at risk of having these

vision condition than the non-dyslexic children although sample size was small

Further research on African children will add to the body of knowledge about this group with

respect to the relationship between vision conditions and dyslexia about which very little is

known Only a few studies have provided evidence of the extent of dyslexia among African

children yet even a small percentage could result in many millions of children being affected

given the population of Africa While the problems that result from dyslexia are not life

threatening they do affect peoplersquos opportunities and quality of life

Unfortunately many African countries do not have the resources to provide specialized

schools for children with learning disabilities and every effort therefore needs to be made to

correctly identify the problems and possible causes of dyslexia In order for African countries

to implement the strategies to meet their millennium development education goals of ensuring

that by 2015 all children will be able to complete a full course of primary schooling they

need to provide not only for children in mainstream schools They need also to plan for

children whose academic ability would be enhanced by the appropriate diagnosis of learning

difficulties the provision of a simple eye test and spectacles or corrective apparatus where

appropriate

Dyslexia is not a disease for which a cure can be found rather long-term sustainable

interventions need to be put in place to ensure that the problems are identified and appropriate

remedial action is taken to minimize the impact it has on their life While many African

countries may not be able to afford schools for children with learning disabilities greater

awareness of the manifestation of the condition needs to be made to all teachers in an effort to

provide these children with the possibility of staying in main stream schools for as long as

possible where there are no alternatives With this in mind it is anticipated that this study will

contribute to the body of knowledge on vision conditions of African dyslexic school children

143

REFERENCES

1 Scheiman M Rouse M 1994 Optometric Management of Learning -Related Vision Problems Missouri Mosby Ist Edition

2 Ygge J Lennerstarnd G Axelsson I Rydberg A Visual Function in a Swedish Population of Dyslexic and Normally Reading Children Acta Ophtahlmol(Copenh)1993 71 (1) 1-9

3 Latvala ML Korhoenen TT Penttinen M Ophthalmic Findings in Dyslexic School Children Br J Ophthal 1994 78 339-343

4 Aasved H Ophthalmological Status of School Children with Dyslexia Eye 1987 161-68

5 Evans BJ Drasdo N Richards IL Investigation of some Sensory and Refractive Visual Factors in Dyslexia Vision Research 1994 34 (14) 1913-1926

6 Buzzelli AR Stereopsis Accommodative and Vergence Facility Do they relate to Dyslexia Optom Vis Sc 1991 68 (11) 842-846

7 Naidoo KS Africa Vision Research Institute Monograph 2007

8 Wu SY Nemesure B Leske MC Refractive Errors in a African Adult Population The Barbados Eye Study Invest Ophthal amp Vis Sci 1999 40 (10) 2179-2184

9 Tielsch JM Sommer A Katz J The Prevalance of Refractive errors Among Adults in the United States Western Europe and Australia Arch Ophthalmology 2004 122 495-506

10 Rudnicka AR Shahrul M Owen C Variations in Primary Open-Angle Glaucoma Prevalence by Age Gender and Race A Bayesian Meta-Analysis Invest Ophth Vis Sc 2006 47 (10) 4254 - 4265

11 Raju D 1997 Poor Oculomotor Control in Dyslexia Masters Degree Thesis University of Durban-Westville

12 Keily PM Crewther SG Crewther DP Is there an Association between Functional Vision and Learning Clin amp Exp Optom 2001 84 (6) 346-353

13 Krupska M Klein C 1995 Demystifying Dyslexia London Language and Literacy Unit

14 Schumacher J Hoffmann P Schma C Schulte G Genetics of Dyslexia The Evolving Landscape J Med Genet 2007 44 289ndash297

144

15 Pushpa S Nallur B Biological basis of Dyslexia A Maturing Perspective Current Science 2006 90 (2) 168-175

16 Sherman G International Dyslexia Association Fact Sheet 969 January 2000

17 Berninger V Dyslexia The Invisible Treatable Disorder Learning Disability Quarterly2000 23 (3) 125

18 Harrie RP Weller C Dyslexia httpwwwericecorg Last accessed January 2007

19 Critchley M Is Developmental Dyslexia the Expression of a Minor Cerebral Damage The William Copeland Memorial Lecture National Hospital London 1966

20 Christenson GN Griffin JR Wesson MD Optometrys Role in Learning Disabilities Resolving the Controversy J of Am Optom Assoc 1990 61 (5) 363-372

21 Evans B 2007 Dyslexia and Vision 2nd Edition Whirr Publishers London

22 Flax N Visual function in Dyslexia Am J Optom Arch Acad Optom 1968 45 574-87

23 Optometric Clinical Practice Guidelines Care of the Patient with Learning-Related Vision Problems America Optometric Association 2000

24 Tan L Spinks JA Eden GF Perfetti CA Siok WT Reading depends on Writing in Chinese Pro Natl Acad Sc 2005 102 (24) 8781 ndash 8785

25 httpfacultywashingtonedu What Is Dyslexia Last Accessed 15th September 2008

26 Shaywitz S Overcoming Dyslexia A new and complete science-based program for reading problems at any level 2003 New York NY Alfred A Knopf

27 Holland KC A look at the Reading Process Optician July 3rd 1987 11-13

28 Bedwell CH The Visual and Ocular aspects of Reading Difficulty Optician Sept 24th 1982 26-38

29 Rayner Keith Eye Movement in Reading Perceptual and Cognitive ProcessesCanadian Psychology Feb 1995

30 Sibylina M Brochure from Learning Disabilities Council of America 1998

31 Pavlidis GT Eye Movements The Diagnostic Key to Dyslexia S Afr OptomJuneAugust 1988 29-39

32 Evans B The Role of the Optometrist in Dyslexia Part 1 Specific Learning Difficulties Optometry Today 2004 January 30 29-33

v

socio-economic status The case group attended a school for children with learning

disabilities while the control group attended a mainstream school At the time of the study

only one school catered for African children with learning disabilities and only 31 of its

pupils were diagnosed with dyslexia Ethical approval was obtained from the University of

KwaZulu-Natal permission to undertake the study in the identified schools was obtained

from the Department of Education and the school principals consented on behalf of the

learners as it was not always possible to reach the individual parent

The researcher (an optometrist) visited both schools by appointment where rooms were

made available to do the testing and the tests were explained to all participants The

LogMar Acuity Charts were used to assess visual acuity and static retinoscopy was used to

assess refractive error Binocular vision was tested using the cover test for ocular alignment

the Hirschberg test for strabismus RAF rule for near point of convergence 2 D flipper

lenses for accommodation facilities Donderrsquos push up methods using the RAF rule for

amplitude of accommodation plus and minus lenses for relative accommodation monocular

estimation technique for accommodation posture and prism bars for vergence reserves

Ocular pathology was assessed using a direct ophthalmoscope

The dyslexic group presented with the following Refractive errors hyperopia 65

myopia 65 astigmatism 10 anisometropia 65 remote near point of convergence

33 esophoria at near 3 exophoria at near 95 accommodative infacility 54 and lag

of accommodation 3928 The dyslexic group had relatively reduced fusional reserve

compared to the control group

The control group presented with the following Refractive errors hyperopia 3

astigmatism 13 anisometropia 65 remote near point of convergence 48 esophoria at

near 0 exophoria at near 0 accommodative infacility 33 and lag of accommodation

4193

The prevalence of a remote NPC was higher in the control group than in the dyslexic group

and there was a statistically significant difference between the two groups NPC break

vi

(p=0049) and recovery (p=0046) The prevalence of poor binocular accommodation

facility at near was higher in the dyslexic group than in the control group and there was a

statistically significant difference between the two groups (p = 0027)

Vision defects such as hyperopia astigmatism accommodation lag convergence

insufficiency poor near point of convergence and accommodative infacility were present in

the dyslexic pupils but they were no more at risk of any particular vision condition than the

control group This study provided the prevalence of vision conditions in a population of

African dyslexic children in South Africa the only vision variable that was significantly

more prevalent in the dyslexic population being the binocular accommodation facility at

near although the study was unable to find a relationship between dyslexia and vision The

statistically significant difference may not imply clinical significance due to the small

sample size However it is recommended that any vision defects detected should be

appropriately compensated for as defective vision can make reading more difficult for the

dyslexic child

The sample size may have been a limitation however this was comparable with studies

reviewed most of which had sample sizes of less than 41 Due to the range of possible

ocular conditions that could affect dyslexia it is recommended that a larger sample size be

used to ensure more conclusive results Testing for relative accommodation with a

phoropter would provide more accurate results and accommodation facility and fusional

reserves would be better assessed with suppression control The study provides information

and an indication of research needs regarding the prevalence of vision defects in an African

South African population of dyslexic children

vii

TABLE OF CONTENTS

CONTENTS PAGETITLE PAGE iDECLARATION iiACKNOWLEDGEMENTS iiiABSTRACT ivTABLE OF CONTENTS viiLIST OF APPENDICES xiLIST OF TABLES xiiLIST OF FIGURES xiiiLIST OF PUBLICATIONS FROM THIS THESIS xiv

CHAPTER ONE INTRODUCTION 1

11 Background and Rationale for the Study 112 Aim 213 Objective 214 Research Questions 215 Significance of study 316 Goals of Study 317 Null Hypothesis 318 Research (alternative) Hypothesis 419 Scope of Study 4110 Assumptions 5111 Study Outline 5112 An overview of Dyslexia 5113 The Reading Process 8114 Eye Movement in reading 9115 Characteristics Features of Children with Dyslexia 10116 Implications of Dyslexia for learning 11

1161 Reading and Comprehension 121162 Handwriting 121163 Proof-reading 12

117 Prevalence of Dyslexia 12118 Aetiology of Dyslexia 13

1181 Neurological Factors 141182 Genetic Factors 151183 Peri-natal Factors 151184 Substance Use during Pregnancy 151185 Environmental Factors 161186 Nutrition 16

119 Diagnosis of Dyslexia 171191 Exclusionary Method 17

viii

1192 Indirect Method 171193 Direct Method 18

120 Functional Imaging Techniques used in the Diagnosis and Study of Dyslexia 191201 Positron Emission Tomgraphy (PET) 191202 Functional Magnet Resonance Imaging (fMRI) 19

121 Classification of Dyslexia 201211 Primary Dyslexia 211212 Secondary Dyslexia 21

122 Sub-types of Dyslexia 211221 Dysphonetic (auditoryphonetic) 211222 Dyseidetic (visual-spatial) 211223 Dysnemkinesia (motor) 211224 Dysphonetic-Dyseidetic 23

123 Theories of Dyslexia 231231 Phonological-Deficit Theory 231232 Cerebellar Theory 251233 Visual Deficit Theory 261234 Auditory Processing Theory 27

124 Cultural Differences in Dyslexia 28125 Treatment Intervention 30126 Role of Optometry in the Multidisciplinary Management of Dyslexia 30

1261 Case History 311262 Refractive and Binocluar Functions 311263 Visual-Motor Skills 311264 Visual-Spatial Analysis 321265 Short-term Visual Memory (visual-motor recall) 321266 Auditory Analysis Skills 32

127 Optometric Treatment Options 331271 Correction of Vision Defects 331272 The use of Coloured Overlay 33

128 Potential Mechanism for Benefit from Tinted Lenses 35129 Summary of Chapter One 37

CHAPTER TWO LITERATURE REVIEW 39

21 Introduction 3922 Visual Acuity 4023 Refractive Errors 47

231 Myopia 47232 Hyperopia 49233 Astigmatism 49234 Amblyopia 50235 Anisometropia 50

24 Near point of Convergence 53

ix

25 Heterophoria 5526 Strabismus 5827 Accommodation Functions 59

271 Amplitude of Accommodation 59272 Accommodation Facility 61273 Relative Accommodation 64274 Accommodation Posture 64

28 Vergence Facility 6529 Fusional Reserves 66210 Ocular Pathology 68211 Summary of Chapter Two 73

CHAPTER THREE METHODOLOGY 75

31 Introduction 7532 Research Design 75

321 Sampling Design 75322 Study population and Participants 75323 Inclusion Criteria for Dyslexic Group 76324 Exclusion Criteria for Dyslexic Group 76

33 Ethical Clearance 7734 Testing Protocol 7735 Study Setting 7736 Data Collection Technique 78

361 Visual Acuity 79362 Refractive Error 79363 Near Point of Convergence 80364 Strabismus 80365 Heterophoria 81366 Evaluation of Accommodation Functions 83367 Fusional Reserves 85368 Ocular Health 87369 Vergence Facility 87

37 Statistical Analysis 8738 Summary of Chapter Three 89

CHAPTER FOUR RESULTS 91

41 Introduction 9142 Prevalence of Vision Defects 93

421 Visual Acuity 91422 Refractive Errors 92423 Near Point of Convergence 94424 Ocular Health 96

x

425 Heterophoria 96426 Accommodation Functions 97427 Fusional Reserves 101

43 Summary of Chapter Four 105

CHAPTER FIVE DISCUSSION 107

51 Introduction 10752 Prevalence of Vision Defects 107

521 Visual Acuity 109522 Refractive Errors 116523 Heterophoria 116524 Near point of Convergence 118525 Accommodation Functions 120526 Fusional Reserves 126527 Ocular Pathology 130

53 Summary of Chapter Five 130

CHAPTER SIX CONCLUSION 132

61 Introduction 13262 Summary of Findings 134

621 Visual Acuity 134622 Refractive Errors 134623 Near Point of Convergence 135624 Heterophoria 135 625 Accommodation Functions 136 626 Fusional Reserves 137627 Ocular Pathology 137

63 Implications of the Study Findings 138 64 The Study Strengths 138 65 Limitations of the Study 139 66 Recommendations 140

661 Future Research 140662 General Recommendations 140

67 Conclusion 141

REFERENCES 143

xi

LIST OF APPENDICES PAGE

APPENDIX A Clarification of Terms 156APPENDIX B Request for Consent from Department of EducationAPPENDIX C Letter of Consent from Department of EducationAPPENDIX D Letter of Consent from Addington Primary schoolAPPENDIX E Visual Acuity Conversion Table

xii

LIST OF TABLES

TABLE TITLE PAGE

11 Characteristic Features of Dyslexia 1012 Features of the Magnocelular and Parvocellular systems 2721 Conclusions from Major Studies on Dyslexia and Vision 7031 Data Collection Tools and Measurement Outcomes 7832 The Diagnostic Criteria for each Test Variable 8741 Descriptive Statistics for Visual Acuity 9242 Descriptive Statistics for Refractive Error 9443 Descriptive Statistics for Near Point of Convergence Break

and Recovery Points 9544 Descriptive Statistics for Heterophoria 9745 Descriptive Statistics for Accommodative Posture 9846 Descriptive Statistics for Binocular Accommodation Facility 9947 Descriptive Statistics for Amplitude of Accommodation 10048 Descriptive Statistics for Relative Accommodation 10149 Descriptive Statistics for Base-in Vergence Reserves at Distance 102410 Descriptive Statistics for Base-in Vergence Reserves for Near 102411 Descriptive Statistics for Base-out Vergence Reserves at Distance 103412 Descriptive Statistics for Base-out Vergences at Near 103413 Descriptive Statistics for Dyslexic and Control Groups

for all Variables 104

xiii

LIST OF FIGURES

FIGURE TITLE PAGE

41 Prevalence of Visual Acuity 9242 Prevalence of Refractive Errors (total) 9343 Prevalence of the Types of Refractive Errors 9444 Prevalence of NPC Break 9645 Prevalence of Phoria at Near 9646 Prevalence of Accommodation Lag 9847 Prevalence of Reduced Binocular Accommodation Facility at Near 9948 Prevalence of Relative Accommodation 101

1

CHAPTER ONE INTRODUCTION

11 Background and Rationale for the Study

Optometry has long been involved in the subject of vision and learning As primary eye care

practitioners optometrists receive referrals from parents teachers psychologists and other

professionals who usually seek the optometristrsquos advice about whether a childrsquos vision problem

may be contributing to or is responsible for poor academic performance

According to Scheiman and Rouse1 learning which is the acquisition of knowledge or skill can

be achieved through study experience or instruction and that between 75 and 90 of what a

child learns is mediated through the visual pathways As vision plays a major role in reading and

the learning process any defects in the visual pathway disrupt the childrsquos learning

ldquoThe three interrelated areas of visual functions are 1

1 Visual pathway integrity including eye health visual acuity and refractive status

2 Visual efficiency including accommodation binocular vision and eye movements

3 Visual information processing including identification and discrimination

spatial awareness and integration with other senses

Several studies2-6 have been conducted to investigate the relationship between vision and dyslexia

on Caucasian populations while very few have been done on African subjects Consequently

most inferences and conclusions on dyslexia are based on research conducted in Caucasian

populations Naidoo7 reported that inferences drawn from studies in Caucasian populations may

not be appropriate for the African population as the lsquoAfrican Eyersquo differs from the Caucasian eye

For example epidemiological studies have reported variations in refractive errors where the

prevalence of myopia is higher among Caucasian persons than in African persons8 9 while

African populations had higher open angle glaucoma prevalence than Caucasian persons10

This study attempts to contribute to the literature by examining the prevalence of a broader

spectrum of vision variables in dyslexic school children in order to investigate a possible

2

relationship between dyslexia and vision in African school children in South Africa A search of

the literature revealed that very little research has been conducted in Africa the only one being

conducted in 1997 by Raju11 Raju11 investigated oculomotor functions in dyslexic school

children in South Africa and reported that 65 of the normal readers exhibited normal

oculomotor control as compared to 11 of the dyslexics Furthermore 54 of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills while 12 of the

controls displayed the same

12 Aim

The aim of the study was to determine the prevalence of vision conditions in a South African

population of African dyslexic children and to study the relationship between vision and dyslexia

13 Objectives

1 To determine the distribution of visual acuity disorders among African dyslexic children

2 To determine the distribution of refractive errors among dyslexic children

3 To determine the distribution of phorias among dyslexic children

4 To determine the distribution of strabismus among dyslexic children

5 To determine the distribution of accommodation disorders among dyslexic children

6 To determine the distribution of vergence disorders among dyslexic children

7 To determine the distribution of ocular pathology among dyslexic population

8 To compare these findings to a similar group of non-dyslexic children

14 Research Questions

1 What is the prevalence of vision conditions among African dyslexic children of age range

10 and 15 years

3

2 What is the relationship between dyslexia and vision

15 Significance of the Study

The study will attempt to

1 Identify high-risk ocular conditions among dyslexic children which might influence the

need for certain examinations or that could affect the childs ability to learn to read

2 Relate findings of this research work to that of previous researchers

3 Add to the knowledge base in the area of vision and dyslexia

16 Goals of Study

1 To obtain quantitative information on the prevalence of visual conditions in dyslexic

children in the African population

2 To obtain information on the relationship between vision disorders and dyslexia

3 To enable improvement of eye care for children with dyslexia

17 Null Hypothesis

1 There will be no statistically significant difference between the mean visual acuity of the

dyslexic group and of the control group

2 There will be no statistically significant difference between the mean refractive errors of the

dyslexic group and of the control group

3 There will be no statistically significant difference between the mean phoria in the dyslexic

group and of the control group

4 There will be no statistically significant difference between the strabismic mean angle of

deviation of the dyslexic group and of the control group

5 There will be no statistically significant difference between the mean accommodation

functions (amplitude facility posture) in the dyslexic group and of the control group

4

6 There will be no statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

1 8 Research (alternative) Hypothesis

1 There will be a statistically significant difference between the mean refractive errors in the

dyslexic group and of the control group

2 There will be a statistically significant relationship between the mean visual acuity of the

dyslexic group and of the control group

3 There will be a statistically significant difference between the mean of phoria of the

dyslexic group and of the control group

4 There will be a statistically significant difference between the strabismus mean angle of

deviation of the dyslexic group and of the control group

5 There will be a statistically significant difference between the mean accommodation

functions of the dyslexic group and of the control group

6 There will be a statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

19 Scope of Study

1 This study is limited to the assessment of visual functions in dyslexic and normal readers

The visual functions investigated include visual acuity refractive error ocular alignment

near point of convergence accommodation functions and vergence reserves The

assessment of eye movement was not part of the study

2 Screening for and diagnosing dyslexia was not part of this study The school

psychologistsrsquo reports were relied on in selecting subjects who qualify to participate in this

study The information was obtained from the learnersrsquo school files 3 The scope of vision parameters investigated is limited only to the prevalence and

relationship between these variables in dyslexic and control groups as outlined in the

hypothesis Correlation analysis of different variables was not part of this study

5

110 Assumptions

In this report dyslexia is used synonymously with ldquodevelopmental dyslexiardquo ldquoreading

disabilityrdquo and reading disorderrdquo and will be used to describe children who find it extremely

difficult to learn to read despite having normal intelligence appropriate educational opportunities

and absence of emotional disorders Children so defined have reading age that is two or more

years behind their chronological age12

111 Study Outline

This thesis is divided into six chapters Chapter one indicates the rationale of the study provides

an overview of dyslexia and details its purpose objectives significance and goals Chapter Two

(the literature review) gives a detailed review of literature in the area of dyslexia learning

disabilities and vision with particular emphasis to vision variables as outlined in the hypothesis

to be tested This is followed by methodology in Chapter Three which details the research

methodology in which a case-control study was done in two schools in Durban The areas of

vision variables evaluated include visual acuity and refraction binocular vision and ocular health

evaluation Chapter Four (Results) presents an analysis of the results obtained from the study

Chapter Five (Discussion) discusses the study findings and their meaning Chapter Six presents

conclusions from the study based on the study results indicating the implications applications

and recommendations for future study and the study limitations

112 An overview of dyslexia

Some children find it difficult to learn to read despite having normal intelligence appropriate

educational opportunities and absence of emotional disorders Their reading age is two or more

years behind their chronological age Such children are referred to as being dyslexics Where

there is an associated deficit in intelligence the condition may be described as generalized

learning disability12

6

The term dyslexia is derived from the Greek words dys meaning hard or difficult and lexia

from the word lexicos which means pertaining to words so dyslexia means difficulty with

words-either seen heard spoken or felt as in writing13

The World Federation of Neurologists13 and the International Classification of Diseases-1014

defined dyslexia as ldquoa disorder manifested by difficulty in learning to read despite conventional

instruction adequate intelligence and socio-cultural opportunity It is dependent upon

fundamental cognitive disabilities which are frequently constitutional in originrdquo

Dyslexia is a complex disability which affects different aspects of reading The acquisition of

reading related skills are coordinated by visual motor cognitive and language areas of the brain

Consequently dyslexia can result from deviation of normal anatomy and function of cognitive

and language areas in the brain 15

Dyslexia refers to a cluster of symptoms resulting in people having difficulties in specific

language skills particularly reading15 The mystery in dyslexia as a syndrome lies in the fact that

dyslexics have ldquodifficulty to learn to read despite having average or above average intelligence

adequate auditory and visual acuity absence of frank brain damage no primary emotional

problems and have adequate educational instructionsrdquo 16

Dyslexia is a developmental disorder that shows in different ways at different developmental

stages As documented by Krupska and Klein 13 dyslexia is a learning disability characterised by

problems in expressive or receptive oral or written language that manifest initially as difficulty in

learning letters and letter sound association which is followed by difficulty in learning letters and

reading words accurately which consequently results in impaired reading rate and written

expression skills (handwriting spelling and compositional fluency) Some dyslexics find it

difficult to express themselves clearly or to fully understand what others mean when they speak

The effects of dyslexia reach well beyond the classroom as it can affect the persons self-image

It makes them feel dumb13 17 Dyslexia describes a different kind of mind often gifted and

productive that learns differently An unexpected gap exists between learning aptitude and

achievement in school 18

Many dyslexics are creative and have unusual talent in areas such as art athletics architecture

7

graphics electronics mechanics drama music or engineering 18 Some dyslexic learn best with

a multi-sensory delivery of language content which employ all pathways of learning at the same

time seeing hearing touching writing and speaking18 It is primarily not a problem of

defective vision although underlying vision deficit has been known to exaggerate the dyslexics

reading problems 18

According to Critchley19 ldquothe child otherwise normal as regards to intellectual status and

emotional stability finds an isolated difficulty in mastering the significance which lies behind

printed or written symbols and of necessity finds it extremely hard to align the verbal

components of speech (as emitted by the mouth and as received by the ear) with the empirical

characters which are set out on paper in one form or anotherrdquo Dyslexics are unable to acquire

written language skills through ordinary learning and teaching methods and often fail to progress

in education19

A dyslexic individual displays a characteristic pattern of decoding (recognising) and encoding

(spelling) difficulties with written words20 Interestingly dyslexics may not have overt

difficulties with spoken language yet do have marked difficulties with written languages21 The

difference between written and spoken language is that written language is seen while spoken

language is heard This fact may explain why vision has always been implicated on the topic of

dyslexia 21

Dyslexics experience difficulties with visual-verbal matching necessary to establish word

recognition ability Even when a clear single visual image is present the dyslexic may be unable

to decode the printed word in some cases This may be related to the fact that the difficulty that

dyslexics have is at a much more fundamental stage in the reading process so the role of vision

function is quite different in dyslexia than in other types of reading difficulty22 (which may be

peripheral such as hyperopia leading to difficulty in reading) The two main situations in which

the term dyslexia now commonly applies are when the reader has difficulty decoding words (that

is single word identification) and the second is encoding words (spelling) The second type is a

frequent presentation in optometric practice that is when the reader makes a significant number of

letter reversals errors (example b-d) letter transposition in word when reading or writing

(example sign for sing) or has left-right confusion 23

8

113 The Reading Process

Language development entails four fundamental and interactive abilities listening speaking

reading and writing while learning to read involves a complex system of skills relevant to visual

(the appearance of a word) orthographic (visual word form) phonological and semantic

(meaning of word and phrases) processing and a variety of behaviors such as letter naming letter

perception word recognition and comprehension each of which uses a different part of the brain

Reading is an interpretation of graphic symbols24-26

For some children the ability to break a word into its smaller parts a task crucial in reading is

extremely difficult Dyslexia is a localized problem one involving the sounds and not the

meaning of spoken language Speaking is natural while reading is different it must be learned

To be able to transform the printed letters on the page to words that have meaning the letters

must be connected to something that already has inherent meaning- the sounds of spoken

language In order to learn to read children must learn how to link the printed letters on the page

to the sounds of spoken language and understand that words are made up of sounds and that

letters represent these sounds or phonemes24- 26 In other words learning to read requires that the

child be able to break the word into the individual components of language represented by letters

and to be able to tell the difference between the individual phonemes that make up a word

Learning to read requires that the central principle behind the alphabet is understood that is

words are made of phonemes and in print phonemes are represented by letters Phonemes are

the shortest units of sound that can be uttered in a given language and that can be recognized as

being distinct from other sounds in the language For example the word cat has three phonemes

ca ah and ta Blended together the sounds form the word rsquocatrsquo24-26 When one speaks the

sounds are blended ttogether and are said one cat 24-26 To learn to read a child must learn that

there are three separate sounds This is difficult for children with dyslexia The inability to break

word into its parts is the main reason why children with dyslexia have trouble learning to

read24- 26

Furthermore dyslexic children find it difficult to bring the print to language (such as when asked

to read what they just wrote) For example a child can copy the letters w-a-srdquo correctly but

9

when asked what was written a child with dyslexia may reply saw The problem in this case

may not be related to vision but rather one of perceptual skills of what the child does with a word

on a page The brain mechanism of going from print to language is phonologically based 24-26

Reading acquisition builds on the childrsquos spoken language which is already well developed

before the start of formal schooling Once a written word is decoded phonologically its meaning

will become accessible via the existing phonology-to-semantics link in the oral language system

Thus the childrsquos awareness of the phonological structure of sound of speech plays a major role in

the development of reading ability 24-25

114 Eye Movement in Reading

For a person to be able to read a letter or word the eye has to first identify the written material

Normally the eyes scan across the line of words on the page stopping to fixate at various points

then making short jumps from one position to the next (some saccade eye movements are

involved) Typically there are around five fixation pauses in a line Sometimes the eye will

regress and go back to a previously passed over word element A reader views the text mainly

with the controlling or reference eye whilst the other eye diverges and converges within fine

elements27 A sophisticated mechanism27 (a connection of the visual receptors of the retina with

the visual nervous pathway to the brain) 28 ensures control of this drift and prevents diplopia from

being consciously observed These drifts and subsequent recovery can be clearly seen by a

number of techniques including infrared observation of the eyes27

The minimum time a person can spend on fixation and move to the next is around 250

milliseconds27 For skilled readers the average saccade length is 7-9 letter spaces and the

duration of saccades is of the order of 25-50 milliseconds29 while the average fixation pauses last

between 200-250 milliseconds27 29 Thus our eyes are still for some 90 of the time while

reading and are subject to large changes in acceleration and deceleration Even a good reader

will regress and go back 10 to 20 of the time27 29

The textual image seen by the eyes is imaged onto both maculae and transmitted to both cerebral

10

cortex where it is first received in the primary visual cortex (Brodmanrsquos area 17) with some

initial processing of colour and tone occur Progressively higher levels of processing occur in

adjacent areas 18 and 19 with the highest level of visual language processing taking place in the

angular gyrus (area 39) The angular gyrus is central to reading since it is here that hearing

speech and vision meets and the written word is perceived in its auditory form The auditory

form of the word is then processed for comprehension in Wernickersquos area as if the word had been

heard25 27

115 Characteristic SignsSymptoms of Children with Dyslexia

Dyslexia is heterogeneous in nature and there is no single pattern of difficulty that affects all

people Dyslexic children frequently show a combination of characteristics (Table 11)

Table 11 Characteristic SignsSymptoms of Dyslexia 16 30-31

Reading Visionminus Holds book too close

minus Word reversals

minus Skips complete words

minus Re-reads lines

minus Points to words

minus Word substitution

minus Sees double

minus Poor comprehension in oral reading

minus Might see text appearing to jump around on a page

minus Unable to tell difference between letters that look similar in shape such as o and e

minus Unable to tell difference between letters with similar shape but different

orientation such as b and p and q

minus Letters might look jumped up and out of order

minus Letters of some words might appear backwards eg bird looking like drib

11

Spelling

minus Omission of beginning or ending letters

minus Can spell better orally than written

minus Letter reversals

minus Wrong number of letters in words

Difficulty with Reading

minus Difficulty learning to read

minus Difficulty identifying or generating rhyming words or counting syllables in

words (Phonological awareness)

- Difficulty with manipulating sounds in words

minus Difficulty distinguishing different sounds in words (Auditory discrimination)

AuditoryVerbalminus Faulty pronunciation

minus Complains of ear problems

minus Unnatural pitch of voice

minus Difficulty acquiring vocabulary

minus Difficulty following directions

minus Confusion with beforeafter rightleft

minus Difficulty with word retrieval

minus Difficulty understanding concepts and relationships of words and sentences

116 Implications of Dyslexia for Learning

Dyslexia affects people in different ways and depends on the severity of the condition as well as

the effectiveness of instruction or remediation The core difficulty is with word recognition and

reading fluency spelling and writing Some dyslexics manage to learn early reading and spelling

tasks especially with excellent instruction but later experience their most debilitating problems

when more complex language skills are required such as grammar understanding textbook

material and writing essays The dyslexic syndrome therefore affects reading and learning in the

12

following ways263034

1161 Reading and Comprehension

One major difficulty dyslexics have is the time it takes to read and understand a text This makes

reading an extremely difficult task and they may report that they get tired and may get headaches

after reading for a short while30 There is also difficulty in assimilating what they have read even

when they know the words Multiple-choice questions are usually difficult for them as they find

it difficult to recognise words out of context263034

1162 Handwriting

Dyslexics have problems with handwriting since for some forming letters takes concentration

that their ideas get lost and their written expression suffers30

1163 Proof-reading

Dyslexics with visual processing difficulties usually have difficulty in identifying errors in

their own writings Reading difficulties make it uneasy for them to see their own errors in

expression and sentence structure grammar26 30

117 Prevalence of Dyslexia

Estimates of the prevalence of dyslexia in Caucasian populations vary from 5 to 20 in the

general population16 Pavlidis 31 reported that about 1 to 3 of the total population of children

suffers from dyslexia and that 20 to 30 of those cases classified as ldquogeneral reading failures

are probably dyslexics Evans32 reported a prevalence of 4 - 5 in children and stated probably

more boys than girls are dyslexic Park33 documented that up to 15 of the school-age

population has some degree of dyslexia with a ratio of four boys to one girl The large variability

in the prevalence reported by different authorities may be attributed to the differences in the

diagnostic criteria and the cut-off point applied to the psychometric tests14 Dyslexia is the most

common and the most researched type of learning disability34

13

A literature search revealed one study in Egypt that reported the prevalence of specific reading

difficulty in 2nd and 3rd grades in elementary school population to be 1 and the male to female

ratio to be 27 to 1 The authors concluded that the prevalence was low compared to that reported

in western countries and that the difference may be due to the way the Arab language is written

and read 35

Contrary to the above reports that males tend to be more dyslexic than females Guerin Griffin

Gottfried and Christenson 36 studied dyslexia subtypes and severity levels with respect to gender

differences The authors36 concluded that

when objective criteria such as the discrepancy between achievement (in

reading andor spelling) and intelligence or direct assessment of coding

skills are used as criterion and non-referred or random samples are

assessed the ratio of males to females identified as dyslexics approaches

11 When identification depends on referrals from parents and or

teachers more males than females may be detected because of the nature

and intensity of boysrsquo behavioural reactions to the disability

According to Shaywitz and Shaywitz34 good evidence based on sample surveys of randomly

selected population of children now indicates that dyslexia affects boys and girls comparably

118 Aetiology of dyslexia

The literature contains diverse theories (not less than four major theories) on the aetiology of

dyslexia probably due to the complex nature of the subject but several authors13-15 26 37-42 have

maintained that it is a neurological disorder with a genetic origin Despite decades of

multidisciplinary research neuropsychology brain anatomy and neuro-imaging 37-42 the specific

causes of dyslexia are still unknown The leading assumption however is that dyslexia

fundamentally stems from subtle disturbances in the brain and other possible explanations of the

cause of dyslexia such as language processing difficulties are based on the neurological etiology

14

The aetiological considerations in dyslexia are

1181 Neurological Factors

As documented by Habib 37 the concept of the neurological basis of dyslexia was first mentioned

independently by a Scottish ophthalmologist Hinshelwood in 1895 and a British physician

Morgan in 1896 Both found similarities of certain symptoms of dyslexia in some children

Dyslexia was then described as ldquovisual word blindnessrdquo The earlier understandings for the

neurological basis of dyslexia came from neuro-pathological studies of brains from dyslexic

persons According to Habib37 it was first reported by the French neurologist Dejerine in

1891 that damage to the left inferior parieto-occipital region (in adults) resulted in variable

degree of impairments in reading and writing suggesting that the left angular gyrus may play a

role in processing the ldquooptic images of letterrdquo It was subsequently thought that impaired reading

and writing in the young dyslexic patients could be due to abnormal development of the same

parietal region which was damaged in adult alexic patients 37

Another perspective 13 37 is that the dense distributions of ectopias (the presence of neural

elements in places where they are not supposed to be found) all over the cerebral cortex

(particularly in the perisylvian language areas) resulted in alteration of brain organisation One

such alteration in dyslexia is the lack of asymmetry in a language-related cortical region called

the planum temporale (an auditory area that lies on the superior surface of the temporal lobe) In

normal subjects the platinum temporale is usually larger in the left hemisphere However the

dyslexics showed symmetry of the planum temporale 13 37 Furthermore these ectopias may

reduce cortical connectivity as suggested by recent positron emission tomography and magnetic

resonance imaging studies 37

Another speculation was that the development of ectopias could be due to foetal hormonal

(possibly testosterone) imbalances during late pregnancy which could also account for the

possible male predominance in dyslexia 15 30 37 38

The maturation lag which is believed to be due to the slower development of the nerve fibres of

the corpus collosum (bundle of cells connecting the left and right hemisphere of the brain) has

also been theorized to be a cause of dyslexia A delay in the growth of the fibres linking the two

15

hemispheres would result in affected children being physiologically incapable of tasks which

require hemispheric organisation This leads to the inadequate development of the language

functions 12 37 However recent neuro-imaging studies 39-42 have shown that impaired reading of

alphabetic scripts (such as in the English language) is associated with dysfunction of the left

hemisphere posterior reading systems primarily in the left temporo-parietal brain regions

1182 Genetic Factors

Genetic considerations in dyslexia are derived from the fact that dyslexia runs most often in

families However it is speculated that different forms of dyslexia may occur within the same

family which also may means that it seems likely that the inheritance is indirect 15 31 However

genetic markers have been identified in chromosome 18 38 and chromosomes 6 and 15 15 In

fact chromosomes 2 (DYX3) 6 (DYX2) and 15 (DYX1) in dyslexics have been reported to be

inherited in an autosomal dominant mode of transmission 15 Furthermore Ramus 43 stated that

ldquogenetically driven focal cortical abnormalities such as ectopias and microgyri in specific areas

of left perisylvian cortex involved in phonological representations and processing are the

primary cause of dyslexia which is consistent with anatomical studies of dyslexic brains showing

loci of cortical abnormalities functional brain imaging studies showing that the very same areas

are involved in phonological processing and show abnormal activation in dyslexicshelliprdquo

1183 Peri-natal Factors

Peri-natal factors may manifest as problems during pregnancy or complications after delivery

More importantly it can also happen that the mothers immune system reacts to the foetus and

attacks it as if it was an infection It has also been theorized (based on partial evidence) that

dyslexia may occur more often in families who suffered from various autoimmune diseases (such

as rheumatoid arthritis Gravesrsquo diseases multiple sclerosis systemic lupus Sjogrenrsquos

syndrome)3137

1184 Substance Use during Pregnancy

16

Drugs such as cocaine (especially in its smokable form known as crack) seem to affect the

normal development of brain receptors of the foetus It has been documented that mothers who

abuse substance during pregnancy are prone to having smaller babies and such infants are

predisposed to a variety of problems including learning disorders Thirdly alcohol use during

pregnancy may influence the childrsquos development and lead to problems with learning attention

memory or problem-solving There is also a risk of foetal alcohol syndrome (characterised by low

birth weight intellectual impairment hyperactivity and certain physical defects) 30

1185 Environmental Factors

Environmental factors are risk factors for the development of reading disabilities 44 Reading

disabilities which are due to environmental influences have been referred to as non-specific

reading disabilities 20 or pseudo- dyslexia as the reading disorder is secondary to extraneous

factors and genetic factors play an important role 45 ldquoEnvironment factors play a significant role

in the level to which neurological impairments manifest themselves Encouraging nurturing and

stimulating surroundings can reduce the impact of risk factors on learning performance and life

skillsrdquo 44 Environmental circumstances which may profoundly influence the dyslexic child

include parental attitude the role of multilingualism in the home attention problems the

drawback of frequent changes of school the personality of the teacher and the childs emotional

reaction to his difficulties 152030

It seems that the influence of environmental factors may best be described as a potential risk

factor than a causative factor in dyslexia as there are diverse views on the relationship between

environmental influences and dyslexia According to Grigerenko46 socioeconomic level

educational opportunity and home literacy level are major environmental risks for the

manifestations of developmental dyslexia hellip the ldquomagnitude of the effects was not strong enough

to view them as powerful causal factorsrdquo

1186 Nutrition

Nutrition plays an important role in bridging the gap between genetic constitution and a childs

potential for optimal development Nutrition is a basic requirement for the maintenance of

17

growth and overall development while malnutrition and under-nutrition are associated with

disorders in the normal development of intelligence perceptual maturation and academic

achievements 47 Studies conducted by Grant Howard Davies and Chasty 48 showed that there

is an association between dyslexia and low concentration of zinc Zinc is necessary for normal

brain development and function and essential for neurotransmission44

119 Diagnosis of Dyslexia

The first stage in the diagnosis of dyslexia is with awareness by parents or educators that a

problem in reading exists The initial point of assessment starts with the medical practitioner

(possibly a paediatrician) who conducts a complete medical examination and obtains a

comprehensive health history A correct diagnosis of dyslexia starts with a very well taken case

history especially good awareness of the childrsquos developmental history This may give

indications of any medical condition that may be contributing to reading difficulties If indicated

the child may be referred for a neurological examination If dyslexia is suspected the physician

may then refer the child for further evaluation and treatment by a specialist in psycho-educational

assessments In most cases the diagnosis of dyslexia is usually performed by the psychologist

The Psychologists conduct psychometric assessments which include assessment of intelligence

quotient (IQ) various aspects of reading performance spelling mathematics sequential memory

and other aspects of cognitive and emotional development49 50 The major purpose of the

diagnostic process is to isolate the specific difficulties associated with dyslexia and to suggest

appropriate educational intervention 49 50

There are three methods for diagnosing dyslexia

1191 Exclusionary Method

In the exclusionary method the diagnosis of dyslexia is made when all factors involved in a

nonspecific reading disability (reading problems resulting from factors such as intellectual

disability dysfunction of hearing or vision inadequate learning experience socio-cultural

deprivation primary emotional problems poor motivation) are excluded 2051 The problem with

18

the exclusionary method of diagnosing dyslexia is that dyslexia cannot be diagnosed until after

the child has been failing in school for almost two years By this time constant failure may have

produced a negative attitude towards school and psychological problems 31

It provides a very

limited description of the features of the disorder Dyslexia is often defined in terms of the

discrepancy between reading achievement and chronological age or grade level failing to

consider the possible overlap between some of the intelligence quotient tests and reading tests 52

According to Snowling 53 IQ is not strongly related to reading and so the use of IQ in defining

dyslexia is inappropriate stating that many children with low IQ can read perfectly well and that

the discrepancy definition (based on the difference between reading achievement and

chronological age) of dyslexia cannot be used to identify younger children who are too young to

show a discrepancy

1192 Indirect Method

The second method of diagnosing dyslexia is the indirect approach A diagnosis is sought by

attempting to associate neurological soft signs such as finger ldquoagnosiardquo with reading failure The

drawback with this method is that many dyslexics do not manifest neurological soft signs 20

Another type of the indirect method involves the analysis of cognitive tests An example is the

Wechsler Intelligence Scale for Children (Revised) (WISC-R) WISC-R which compares ldquoverbalrdquo

versus ldquoperformancerdquo 20

1193 Direct Method

The third method which seems most reliable is the direct method which involves the use of

characteristic decoding and spelling patterns to determine the specific dyslexia It has the

additional advantage of addressing the fact that dyslexia is heterogeneous 20

19

120 Functional Imaging Techniques used in the Diagnosis and Study of

Dyslexia

There are functional imaging techniques for diagnosing and studying brain activity in dyslexia

These techniques include positron emission tomography (PET) Magnetic resonance imaging

(MRI) and computed tomography (CT) The two major techniques include

1201 Positron Emission Tomography (PET)

PET is a non-invasive imaging technique that uses radioactively labeled compounds to

quantitatively measure metabolic biochemical and functional activity in living tissue It

assesses changes in the function circulation and metabolism of body organs 54 55 The PET is

a sophisticated nuclear medicine technique that looks directly at cerebral (brain) blood flow

and indirectly at brain activity (or metabolism) The most commonly used labels are

flourophores which emit light when stimulated with light of the appropriate wavelength and

radio nuclides which emit gamma rays or beta particles when they decay56 The dose of

radionuclide injected is minute and does not pose any significant hazard Various isotopes are

used depending upon the brain region and function studied54

Unlike MRI or CT imaging which primarily provide images of organ anatomy PET measures

chemical changes that occur before visible signs of disease are present on CT and MRI images 54 While PET is most commonly used in the fields of neurology oncology and cardiology

applications in other fields are currently being studied 57

1202 Functional Magnetic Resonance Imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a noninvasive usually painless procedure that

uses a powerful magnetic field radio waves and a computer to produce detailed pictures of

organs soft tissues bone and virtually all other internal body structures The images can then be

examined on a computer monitor or printed The fMRI does not use ionizing radiation57 58

Functional magnetic resonance imaging (fMRI) is a relatively new procedure that uses MR

imaging to provide a picture of the brains active rather than its static structure The fMRI

20

machines measure the level of oxygen in the blood through a technique called blood level-

dependent (BOLD) contrast The two different states of hemoglobin-oxygen-rich oxyhemoglobin

and oxygen-poor deoxyhemoglobin differ in their magnetic properties The huge fMRI magnets

are sensitive to changes in the concentration of deoxyhemoglobin As neural activity increases

blood flow to the vasculature of the brain presumably increases altering this concentration The

more active a brain area is the more blood flows to it Thus fMRI can provide a moment-by-

moment movie of brain activity

Studies using fMRI are beginning to shed light on dyslexia Functional magnetic resonance

imaging (fMRI) is similar to MRI as both procedures use a similar scanner The difference is that

the fMRI uses somewhat more sophisticated hardware and software that allow it to capture brain

changes (mainly blood flow) as a person performs a specific cognitive task The functional

magnetic resonance imaging (fMRI) scanner measures where blood flows through the brain

during certain tasks26

Earlier studies of dyslexia were based on histo-pathological studies According to Habib37 the

fMRI has an added advantage that cortical anatomy of the dyslexic brain can be demonstrated

thus answering the criticism addressed to results from pathological findings

120 Classifications of Dyslexia

Dyslexia can be classified into two broad categories according to its presumed aetiology The

following methods of classification have been documented by Helveston59

1 Primary dyslexia (specific developmental dyslexia)

2 Secondary dyslexia

1211 Primary Dyslexia

This is considered to be caused by a specific central nervous system defect This neural defect is

thought to be located in the angular gyrus of the dominant hemisphere It is often hereditary and

21

may affect more boys than girls In primary dyslexia the decoding process involved in language

system is affected and reading becomes extremely difficult for the child However intellectual

tasks that do not involve reading text are unaffected 59

1212 Secondary Dyslexia

There are two types of secondary dyslexia59 Endogenous and Exogenous

a Endogenous Dyslexia this results from pathological changes in the central nervous

system secondary to trauma or disease such as childhood meningitis hydrocephalus

with brain damage and porencephalus59 Other medical causes include prematurity

congenital hydrocephalus encephalitis traumatic brain injury and lead or

methylmercury poisoning50

b Exogenous Dyslexia this results from intellectual deprivation usually limited to lack of

adequate educational opportunity or motivation59 or due to low intelligence socio-

cultural deprivation primary emotional problems sensory impairment (visual or

auditory) poor motivation or attention problems20 Reading disability due to these

mentioned factors have been referred to as non-specific reading disability20

1 22 Sub-types of Dyslexia

When children first start to read they learn to recognise simple words by their shapes (sight

analysis) building their sight vocabulary This is followed by learning how to analyse

complicated words by breaking them down into their sound components (phonetic analysis)

The dynamics of the two processes (Sight-analysis and Phonetic analysis) are important in

explaining how words are decoded based on the neurologic-behavioural model documented by

Christenson et al 20 and Evans 21 Dyslexia can be divided into the following subtypes 20 21

22

1221 Dysphonetic (auditoryphonetic)

The individual has a minimal dysfunction involving the Wernickes area Readers have an

extremely difficult time developing phonic word analysis or decoding skills Their typical

reading errors are wild guesses (lsquolikersquo for lsquolittlersquo) errors picking words with similar shapes

(horse for house ) and semantic substitutions (home for house) Their typical spelling errors

are phonetically inaccurate (reffor rough) This type of reader persists in using whole-word

approach and will guess at unfamiliar words rather than using their word analysis skills While

they do not often continue to reverse letters and words after the age of eight they often make

spelling reversals (For example interver for inventor and wirters for writers) Typically

they often look at the first and last letter and the length of the word and then guess (monkey for

money or stop for step)20 21

1222 Dyseidetic (visual-spatial)

In dyseidetic type of dyslexia dysfunction occurs in the angular gyrus The individual have high

ability in phonic word analysis They often have a difficult time learning the letters of the

alphabet They usually have a very low sight vocabulary poor visual memory poor perceptual

skills and letter and word reversals (dig for big or saw for was) Their reversals often last

past the age of eight and include both reading and writing reversals This type of child is often a

very slow reader who often will try to sound out familiar as well as unfamiliar words They spell

the word as it sounds By first identifying the learning style of these children probably during the

case history the optometrist may know the childs strengths and weaknesses Vision therapy can

then be designed to fit each childs needs Enhancing the childrsquos visual perceptual and eye

tracking skills plus having the schools teach these children according to their strengths enable

them to achieve their full potential in school 20 21

122 3 Dysnemkinesia (motor)

Dysnemkinesia is a less serious form of dyslexia and it involves minimal dysfunction of the area

of the motor cortex involved in letter formation Dysnemkinesics can be distinguished by their

23

frequent letter reversals such as d for b as in doy for boy Additional dyslexic types occur

when combinations of the three basic types arise such as dysphoneidesia dysnemkineidesia

dysnemkinphonsia and dysnemkinphoneidesia20 21

1224 Dysphonetic-Dyseidetic

Dysphonetic- dyseidetic reader has a weakness in both whole-word and phonic analysis Their

reading errors include wild guesses (fish for father) and word reversals (no for on) Their

spelling is usually phonetically inaccurate and often bizarre (rit for faster) 2021

123 Theories of Dyslexia

Assisted by the increasing body of knowledge obtained by neurophysiological and imaging

studies14 several theories have been proposed with the aim of characterizing the fundamental

processes underlying dyslexia14 These theories can be classified under four major frameworks

phonological visual cerebellar and auditory

1231 PhonologicalndashDeficit Theory

According to the phonological theory affected individuals have difficulties in perceiving and

segmenting phenomes leading to difficulties in establishing a connection between phonemes and

graphemes 14 The brain recognizes language in a hierarchical order The upper levels deal with

semantics (the meaning of words) syntax (grammatical structure) and discourse (connected

sentences) The lower levels of hierarchy deal with breaking sounds into separate small units

called phonemes (sound units) Thus before words can be comprehended at higher levels in the

hierarchy it has to be broken down into phonologic constituents that the alphabetic characters

represents15 26 An adequate reading development stems from some considerable level of spoken

language already acquired by a child in the early years of life Consequently failure to develop

the association between letters (grapheme) and sound (phoneme) has been considered to be a

major cause of reading and spelling impairment in most cases of developmental dyslexia60-62

24

Reading requires some skills of phonological processing to convert a written image into the

sounds of spoken language1 According to the phonologic-deficit hypothesis people with

dyslexia have difficulty developing an awareness that words both written and spoken can be

broken down into smaller units of sound and that in fact the letters constituting the printed word

represent the sounds heard in the spoken word2

According to Habib37 neuropsychological studies have provided considerable evidence

indicating that the main mechanism leading to reading difficulties is phonological in nature and

that it has been repeatedly demonstrated that the core deficit responsible for impaired learning to

read is phonological in nature and has to do with oral language rather than visual perception At

the brain level this cognitive deficit would arise from a congenital dysfunction of certain cortical

areas involved in phonology and reading60 At the neurological level it is usually assumed that

the origin of the disorder is a congenital dysfunction of the left-hemisphere perisylvian brain

areas underlying phonological representations or connecting between phonological and

orthographic representations60

Brain imaging studies in dyslexics in response to a phonological task indicate under-activation of

posterior brain regions (Wernickesrsquos area angular gyrus and striate cortex) and relative over-

activation in anterior regions (inferior frontal gyrus) These brain activation patterns provide

evidence of an imperfectly functioning brain system for segmenting words into their phonologic

constituents39 42 Since most dyslexics show deficits in phoneme processing the phonological

deficit is the most significant consistent marker of dyslexia and remains the predominant theory15343739 hence the focus on phonology in the majority of remedial programmes 63

Based on the phonological deficit theory the other symptoms of dyslexia are considered as

simple co-morbid markers that do not have a causal relationship with reading disability62

However proponents of the phonological theory typically contend that these disorders are not

part of the core features of dyslexia62 A major setback with the phonological theory is that it

does not account for symptoms such as impairment in visual perception and problems with motor

coordination that are found in dyslexics 15 and does not account for symptoms of dyslexia that are

unrelated to phonetic decoding difficulties such as problems with short term memory and visual

25

processing issues64

The two basic processes involved in reading are decoding and comprehension In dyslexia a

deficit at the level of the phonologic units impairs the ability to segment the written word into its

underlying phonologic elements Consequently the reader experiences difficulty initially in

decoding the word and then in identifying it The phonologic deficit is independent of other non-

phonologic abilities especially the higher-order cognitive and linguistic functions involved in

comprehension such as general intelligence and reasoning vocabulary and syntax are generally

intact A deficit in phonologic analysis and an ironical intact higher-order cognitive ability offers

an explanation for the paradox of otherwise intelligent people who experience great difficulty in

reading 26

According to the phonological model a deficit in a lower-order linguistic (phonologic) function

prevents access to higher-order processes and to the ability to draw meaning from text The

problem is that the person cannot use his or her higher- order linguistic skills to access the

meaning until the printed word has first been decoded and identified Suppose for example that

a man knows the precise meaning of the spoken word apparition however until he can decode

and identify the printed word on the page he will not be able to use his knowledge of the

meaningof the word and it will appear that he does not know the words meaning 26

1232 Cerebellar Theory

The normal pattern of cerebellar asymmetry is anomalous in dyslexia 13 The cerebellar deficit

theory suggests that the automatisation of cognitive processes and motor control in the

cerebellum are disturbed in individuals with dyslexia14 15 62 Evidence for this theory comes

from poor dyslexic performance in coordination balance and time estimation The cerebellar

theory is related to cerebral dominance (greater proficiency of each cerebral hemisphere in

acquisition and performance of certain specific functions) and lateralization (one-sided location

of brain function)13

Physiologically the brain operates cross-laterally thus the left part of the brain controls the right

side of the body and vice versa The human brain is normally asymmetrical due to the

26

specialized functions in the left and right brain However the dyslexic brain appears to be more

symmetrical than the normal brain The consequence of this is that both sides of the brain may

be competing to handle language and therefore less efficient in handling processing language

The left hemisphere is especially important for the production of language The right hemisphere

also has some language capabilities but has no ability to analyse speech sound Secondly the

cerebellum plays a role in motor control and therefore in speech articulation It is postulated that

retarded or dysfunctional articulation would lead to deficient phonological representations

Furthermore the cerebellum plays a role in the automatization of tasks such as driving typing

and reading A weak capacity to automatize would affect among other things the learning of

grapheme-phoneme correspondences 13-15 62

1233 Visual Deficit Theory

For many years a major view was that reading difficulties are caused by dysfunction in the

magnocellular subsystem 65-67 The visual theory does not exclude the phonological deficit but

emphasizes a visual contribution to reading problems in some dyslexics The biological basis of

the proposed visual theory is derived from the division of the visual system into two sub-systems

which carry information to the visual cortex the transient and the sustained systems62

Anatomically the magnocellular pathway receives input from both rods and cones across the

retina and extends from ganglion cells in the retina to the two ventral layers of the lateral

geniculate nucleus (LGN) The large cell bodies in these two layers give the pathway its name It

projects via the visual cortex and the dorsal or ldquowhererdquo stream to the parietal cortex The

parvocellular (or sustained) system on the other receives input from cones and therefore mainly

the central retina and runs from retinal ganglion cells to the smaller cell bodies of the four dorsal

layers of the LGN It projects via the visual cortex and the ventral or ldquowhatrdquo stream to the

infero-temporal cortex and mediates colour vision and detection of fine spatial details61 The

magnocellular system is the first to respond to a stimulus and relies on further detail from the

parvocellular system (Table 12)

27

Table 12 Features of the Magnocellular and Pavocellular Systems2168

Magnocellular(transient) System Pavocellular (sustained) SystemLarge cells Small cells

Dim light Bright light

High contrast sensitivity Low contrast sensitivity

Low spatial frequency High spatial frequency

Peripheral Central

Right brain Left brain

Midbrain Cortical

Non-cognitive Cognitive

Emotional Logical

Hyperopia Myopia

Gross depth perception Stereopsis

Achromotopsia Colour vision

Fast transmission Low transmission

Sensitive to large stimuli Sensitive to small stimuli

The transient system is vital to the timing of visual stimuli and plays an important part in

detecting the movement of objects (motion perception) and ultimately their signals are used to

control eye and limb movements made in relation to visual targets1521 Impaired function of

magnocellular pathway will lead to destabilization of binocular fixation which results in visual

confusion and letters then appear to move around

Dyslexicsrsquo eyes are unsteady when they are attempting to view small letters hence their vision is

unstable they tend to make visual reading errors15 There is a tendency to superimpose letters on

top of each other and mis-sequence letters in a word1621 Evidence for magnocellular

dysfunction comes from anatomical studies of dyslexic individuals showing abnormalities of the

lateral geniculate nucleus1562 psychophysical and neuroimaging studies 62 64 showing decreased

sensitivities in the magnocellular range (low spatial frequencies and high temporal frequencies)

in dyslexics

28

A deficit in the magnocellular system slows down the visual processing system resulting in an

unstable binocular control inaccurate judgements of visual direction a tendency to superimpose

letters and mis-sequence of letters in a word16 21 Some studies65-67 have shown evidence in

support of the magnocellular deficit theory of dyslexia

1234 Auditory Processing Theory

The auditory processing theory specifies that the deficit lies in the perception of short sounds and

fast transitions (called the ldquorapidrdquo or ldquotemporalrdquo auditory processing deficit) Such a

characterization of the auditory dysfunction is consistent with the magnocellular theory since

magnocellular cells are particularly sensitive to high temporal frequencies60 62

Proponents of the auditory processing theory hypothesized that impaired perception of brief

sounds and transitions would be particularly detrimental to speech perception hence would

undermine the development of the childrsquos phonological representations Contrary to this

hypothesis Rasmus60 reported that recent studies have now adequately confirmed that there is

no reliable relationship between performance on rapid auditory processing tasks and speech

categorization and discrimination neither is there a reliable relationship between any auditory

measure (speech or non-speech) and more general measures of phonological skill or reading

ability even when assessed longitudinally A possibility is that the most auditorily impaired

dyslexics also have severely impaired phonology and reading although the reverse is not

necessarily true 60

Despite the fact that evidence exist in support of the theories and that some of the deficits found

in affected individuals are correlated with reading and spelling they may not be causally

associated with dyslexia However results from genetic research may have the potential to help

delineate which cognitive and neurophysiological processes are causally related14

124 Cultural Differences in Dyslexia

The recognition of dyslexia as a neuro-developmental abnormality has been influenced by the

belief that it is not a specific diagnostic entity because it has variable and culture-specific

29

manifestations 69-71 Recent neuro-imaging studies39-42 have demonstrated that impaired reading

of alphabetic scripts is associated with dysfunction of left temporo-parietal brain regions

However it has been reported by Siok et al in two separate studies69 70 that in contrast to the

assumption that dyslexia in different languages has a universal biological origin their study using

functional magnetic resonance imaging with reading-impaired Chinese children and associated

controls showed that functional disruption of the left middle frontal gyrus is associated with

impaired reading of the Chinese language (a logographic rather than alphabetic writing system)

The neural connections involved in reading and reading disorders may vary in different languages

because of differences in how a writing system links print to spoken language69 70 In Chinese

for example graphic forms (characters) are mapped to syllables which differ markedly from an

alphabetic system (such as English) in which graphic units (letters) are mapped to phonemes

These differences can lead to differences in how reading is supported in the brain69 70 Readers

of Chinese show relatively more engagement of visuo-spatial areas and left middle frontal

regions for verbal working memory presumably for recognizing complex square-shaped

characters whose pronunciations must be memorized instead of being learned by using letter-to-

sound conversion rules69 70

In two separate studies Siok Perfetti and Tan69 and Siok Zhen Charles Perfetti and Tan 70

studied language-related activation of cortical region of Chinese dyslexic school children using

the functional magnetic resonance imaging and found a reduced activation in the same left

middle frontal gyrus region in Chinese dyslexics Chinese dyslexics by contrast did not show

functional or structural differences from normal subjects in the more posterior brain systems that

have been shown to be abnormal in alphabetic-language dyslexics69-70 The authors concluded

that the results of the studies suggested that the structural and functional basis for dyslexia varies

between alphabetic and non-alphabetic languages and that the findings provided an insight into

the fundamental patho-physiology of dyslexia by suggesting that rather than having a universal

origin the biological abnormality of impaired reading is dependent on culture

The study by Paulesu Demonet Fazio and McCrory71 gave another perspective Using the

positron emission tomography to study brain activity of dyslexics from three countries (Italian

French and English) the authors found the same reduced activity in a region of the left

30

hemisphere in dyslexics from all three countries with the maximum peak in the middle temporal

gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus

Their study showed that there was a universal neuro-cognitive basis for dyslexia and that

differences in reading performance among dyslexics of different countries are due to different

orthographies Languages with transparent or shallow orthography (such as Italian) the letters

of the alphabet alone or in combination are in most instances uniquely mapped to each of the

speech sounds occurring in the language Learning to read in such languages is easier than in

languages with deep orthography (such as English and French) where the mapping between

letters speech sounds and whole-word sounds is often highly ambiguous 71

125 TreatmentIntervention

Individualized instruction of dyslexics emphasizing increased phonologic awareness can have a

favorable long-term effect on academic achievement Individualized instruction aimed at

increasing phonologic awareness decoding skills sight word vocabulary and reading

comprehension are particularly helpful to school aged children experiencing difficulties with

reading Most importantly intervention (provided by expert teachers) should be initiated early50

Reading disabilities require lifelong assistance and optimal management strategies differ

depending on the patientrsquos age and circumstances In early childhood the focus is on

remediation of reading often with an emphasis on increasing phonologic awareness Other

strategies include using audio books and modified homework assignments For secondary and

college students intervention focuses on accommodations which include extra time for reading

tape recorders in the classroom audio books instruction in word processing and the use of a

spell-checker (poor phonemic association also causes problems in spelling)3450

126 Role of Optometry in the Multidisciplinary Management of

Dyslexia

The role of the optometrist in the multidisciplinary management of dyslexia starts with an attempt

31

to identify characteristics which may indicate that the child may be dyslexic A complete visual

examination should be conducted Any vision anomalies detected should be corrected and a

prompt referral should be made to the appropriate specialist in the diagnosis of dyslexia and a

follow up should be made by the optometrist

A comprehensive visual evaluation should include1 72

1 Evaluation for a sensory problem or refractive anomaly

2 Evaluation for binocular vision problems

3 Evaluation for ocular pathology

4 An evaluation of visual motor skills (required for drawing and handwriting) eye movement

control skills and visual imagery skills

5 Evaluation of visual information processing including visual spatial skills

(rightleft discrimination) visual analysis skills (matching and discrimination skills)

The role of the optometrist in the management of reading dysfunctions is to identify existing

vision disorders which can be achieved by evaluating the following aspect of vision functions

1261 Case History

A detailed case history is paramount for children who present with reading dysfunction A

thorough investigation of the pre-natal peri-natal and post-natal risk factors is important to

establish the possibility of major developmental delay For example maternal history of

alcoholism may indicate foetal alcohol syndrome Prematurity (early birth date of less than 37

weeks or low birth weight of less than 2500g) is also a factor in later learning A working

knowledge that may not necessarily mean expertise in all related areas may be adequate for the

optometrist72

1262 Refractive and Binocular Functions

A thorough investigation and compensation for refractive and binocular vision functions should

be undertaken4472

1263 Visual-Motor Skills

32

VisualndashMotor integration is the ability to coordinate visual information-processing skills with

motor skills44 It is a very vital skill in academic achievement4472 Children suffering from visual-

motor dysfunction have difficulty copying written work accurately and efficiently Difficulty

with copying can reduce the speed and accuracy of writing which subsequently affects

learning4472 The Beery Test for Visual-motor Integration is particularly useful for the primary

care optometrist It requires only a few minutes to administer with an added advantage that the

scoring guide provides many examples of the pass-fail criterion for each item tested72

1264 Visual-Spatial Analysis

ldquoThe visual-spatial skills allow the child to make judgments about location of objects in visual

space in reference to other objects and to the individuals own body It develops from awareness

within the individualrsquos body of concepts such as left and right up and down and front and back It is important for good motor coordination balance and directional sensesrdquo 72 The Rosnerrsquos

Visual Analysis Skills programme is useful for the assessment of both visual-motor skill and

spatial analysis72

1265 Short- Term Visual Memory (Visual-Motor recall)

The visual memory test assesses the ability to recognize and recall visually presented material 4473 The ability to copy written material accurately is more or less a matching process when the

written materials are close (such as copying from workbook to paper) When the reading

material is separated from the actual visual-motor task such as copying from chalkboard to

paper the process becomes more complex An example of short-term visual memory skill

assessment test is the Getman-Henderson-Marcus Test of Visual recall72

1266 Auditory Analysis Skills

Screening for auditory-perceptual skill at the primary care level is essential as the ability to

analyze spoken language into separate sounds and sound sequence is directly related to reading

and spelling success Examples of tests to measure auditory perceptual skills are the Test of

33

Auditory Analysis Skills (TAAS) (which is easy to administer and score) and a more complex

one Test for Auditory-Perceptual Skills72

127 Optometric Treatment Options

The optometric treatment options for the correction of vision defects include the following

options

1271 Correction of Vision Defects

1 Compensation for refractive error

2 Treatment of visual efficiency deficits with lenses prisms and vision therapy

3 Treament of vision information processing deficits with vision therapy commencing with

visual spatial orientation then visual analysis and concluding with visual-motor

integration

4 Referral to another health care professional educational system and or psychologist

should be considered at any time if underlying physical or neurological problems cognitive

deficits or emotional disorders are suspected1 50 72

The expected outcome of an optometric intervention is an improvement in visual function with

the reduction of vision anomalies associated with reading which makes educational

intervention easy1 Solan47 proposed that the role of optometrists should not end with

correcting vision anomalies in dyslexic patients but that optometrists should participate in

community services to assist with the developmental environment of those whose genetic

make-up may predispose them to having reading difficulties

1272 The Use of Tinted Lenses and Coloured Overlays

The patients rely on professionals to attend to their vision care needs Consequently it is

imperative that we practice at the highest level of professionalism and ensure that anything we

prescribe for our patients are supported with credible research

As reported by Evans74 75 Olive Meares in 1980 suggested that some childrenrsquos perception of

text and certain cases of reading difficulties were influenced by print characteristics which

34

subsequently resulted in visual perceptual anomalies such as words blurring doubling and

jumping Subsequently Meares and Irlen proposed that ldquoScotopic sensitivity syndrome or

(Meares -Irlen syndrome) ndash a syndrome of visual symptoms and distortion can be alleviated

with coloured filters Meares and Irlen claimed that the coloured overlays improve reading

ability and visual perception increase sustained reading time and eliminate symptoms

associated with reading such as light sensitivity eyestrain headaches blurring of print loss of

place and watery eyes75 This type of symptoms is reported both by people who experience

frequent severe headaches of the migraine type as well as some people with dyslexia76

Following this initiative Irlen developed a proprietary system in countries like the United States

of America United Kingdom and Australia to detect and treat Meares-Irlen syndrome Meares-

Irlen syndrome is reported to be treated with coloured filters either coloured sheets (overlays)

placed on the page or colored lenses Computer users may attain a similar effect by changing the

colour of the screen background and or text while people that consistently work under the same

lighting conditions can benefit from the colour therapy by simply varying the colour of the

illuminating light75

The earlier studies of Irlen was criticized for its lack of published double masked studies with a

placebo control to support her claims that the colour that helped had to be specific for each

person76

According to Lightstone76 the Intuitive Colorimeter (developed by Professor Wilkins) of the

Medical Research Council in the United Kingdom was the first to be used to conduct a masked

trial With this instrument it was possible to determine the optimum colour for the spectacle lens

without placing lenses in front of the eyes and could be done in a way that the subject is unaware

of the exact colour they had selected At the end of the study it was felt that the use of colour

was credible and was then included as part of a routine assessment of people with specific

learning difficulties at the Institute of Optometry London76

As described by Lightstone76 the Intuitive colorimeter is basically a large box which the patient

or subject looks into The space that the subject looks at is entirely diffused by a coloured light

and so creates the effect of wearing coloured lenses There are no other colours in the field of

35

view and so no reference colours are available to help the subject decide the exact hue (colour) of

the light that they are viewing This encourages rapid colour adaptation The person operating

the colorimeter changes all aspects of the colour that the subject is viewing and the hue

saturation (depth of colour) and the brightness of the colour can be varied independently of each

other Although the procedure is subjective the technique shows whether there is a specific

colour that gives relief from the symptoms If the effect is placebo the results tend to be variable

and no specific colour can be selected as the optimum colour75 76

An advantage of the Intuitive colorimeter is that because of colour adaptation the person is

unaware of the exact color of the light shining on the text74 However a major demerit with

double-masked trial was that the results were purely subjective and there were no objective

measurement of any improvement with the colour 76

128 Potential Mechanism for Benefit from Tinted Lenses

Some theories have been proposed to be the mechanism of benefit from tinted lenses However

Evans 49 suggested that these hypotheses have not been able to account for the high degree of

specificity of the required colour which has been emphasized by Irlen and substantiated by

double-masked randomized placebo-controlled trials

The theories are

a Pattern Glare The basis of the theory on how pattern glare explains the benefits of tinted

lenses is that certain cells within the visual cortex are hypersensitive and so causes

discomfort while viewing patterns of repeating black and white lines on a page 75 Since

some of the neurons in the visual cortex are sensitive to specific colours varying the colour

of the illuminating light may change the pattern of excitation within the cortex This could

account for the benefit from specific coloured filters From this it has been hypothesized

that by changing the colour of the input the stimulus is transmitted to other cells that are

not hypersensitive and so prevent the distortions and discomfort occurring 7576

b Syntonics Syntonics is a colour vision therapy system used by some optometrist for the

treatment of several conditions including strabismus amblyopia and reading disorders

36

The therapy involves subjects viewing a light source covered by a colored filter The theory

is that the colour influences the endocrine system through the inferior accessory optic tract

The colour seems to be chosen on the basis of a hypothetical connection between an alleged

autonomic nervous system imbalance and the type of heterophoria Treatment is said to

result in improved visual fields7475

c Placebo Effect The placebo effect is one of the possible mechanisms for the benefit from

tinted lenses People may believe that a treatment works simply because it is claimed to be

effective74

Williams and Kitchner77 conducted a literature review on the use of tinted lenses and colored

overlays for the treatment of dyslexia and other related reading and learning disorders and

concluded that

1 there was evidence that the symptoms associated with Irlen syndrome are related to

identifiable vision problems such as binocular and oculomotor problems and that condition

returned to normal when treated with lenses prisms or vision therapy When patients

exhibiting the Irlen syndrome were treated with vision therapy the symptoms were relieved

and therefore did not demonstrate the need for colored lenses

2 most investigators did not control for the presence of vision anomalies

3 the results of prospective controlled research on the effectiveness of tinted lenses or colored

overlays varied

4 the results of testing used to determine the most appropriate color were not repeatable

5 the effects of spectral filters and colored overlays were not solely a placebo Colored

overlays and tinted lenses are not cures for dyslexia but may be useful reading aids for

some individuals with reading difficulty

6 there was a lack of agreement about the best ways to evaluate patients for the presence of

Irlen syndrome

The American Optometric Association78 issued a policy statement on the use of tinted lenses and

colored overlays which states as follows

37

1 Undetected vision problems may be a factor in individuals who exhibit the symptoms of

the Irlen Syndrome A comprehensive eyevision examination with particular emphasis

on accommodation binocular vision and ocular motor function is recommended for all

individuals experiencing reading or learning difficulties as well as those showing signs

and symptoms of visual efficiency problems

2 The American Optometric Association encourages further research to investigate the

effect that specifically tinted lenses and colored overlays have on visual function related

to reading performance

3 Vision problems are a frequent factor in reading difficulties Ignoring the role of vision

or inadequately evaluating the vision of individuals with reading problems is a disservice

which may prevent the person from receiving appropriate care

In concluding Lighthouse76 advices that until more is known about prescribing specific colored

lenses optometrists should be certain to offer the patient the best advice According to Wilkins 79

some 20 of unselected children in mainstream education (not simply those with reading

difficulties) find colored overlays of benefit and the benefit is pronounced in 5 Wilkins79

emphasized that ldquocolored filters do not provide a treatment for dyslexia ndash they provide a means of

avoiding the visual stress with which reading is sometimes associatedrdquo

129 SUMMARY

Dyslexia is a neuro-developomental condition that impacts on a persons spelling

comprehension writing and reading and not only limits their academic career but also impacts

on all areas of life that require these skills Its diagnosis management and treatment require a

multidisciplinary team as it can present with many characteristics which requires careful

classification While it is not always possible to identify the cause it is generally considered to

be a neurological condition with a genetic origin It affects up to one fifth of Caucasians with

limited studies being done on other ethnic groups including Africans

The aim of the study was therefore to investigate the prevalence of vision defects in an African

38

South African population of dyslexic schoolchildren with an attempt to identify high risk

visual conditions that may be prevalent It was hypothesized that there was no relationship

between vision defects and dyslexia

The reading process requires learning to transform printed letters on a page to words that have

meaning Words are made up of phenomes which are represented by letters and it is the

inability to break words into parts that result in the manifestation of dyslexia Its characteristics

include letter reversal faulty pronunciation difficulty with word retrieval and learning to talk

As all visual stimuli is transmitted through the eye there is an association between ocular

conditions and problems associated with letter words and reading hence the involvement of

optometrists The primary role of optometry is to identify and compensate for visual defects

that may constitute an impediment to reading Evaluation by an optometrist is therefore

important to assess and provide advice or devices where appropriate

Initial identification usually takes place by teachers or parents and may result in the child

being sent to a school equipped to deal with learning disabilities Psychologists diagnose

dyslexia using some range of psychometric tests The advent of imaging techniques such as

positron emission tomography and functional magnetic resonance has aided research in

dyslexia as these techniques enable the study of dyslexic brain activity as a reading task is

performed The two broad categories of dyslexia one being due to a genetic neurological

defect and the other being due to a secondary incident that impacts on the brain There are a

number of theories regarding the underlying causes the only theory related (still controversial)

to visual system being the visual processing theory

Chapter One provided the study aims objectives hypotheses to be tested background

information on dyslexia and rationale for the study while Chapter two will review the related

literature on dyslexia and vision

39

CHAPTER TWO LITERATURE REVIEW

21 Introduction

The relationship between vision and dyslexia has been a subject of controversy in optometric

and ophthalmologic literature A review of existing literature 1-32 on the area of vision and

dyslexia reveals that the studies conducted to investigate visual functions in the dyslexic

subjects do not establish a causal relationship between vision and dyslexia but tend to

establish associations between dyslexia and certain vision variables The literature review also

shows that there exists a dichotomy between the ophthalmologists and optometrists on the

subject of dyslexia and vision The difference in perspective can be seen from the style of

report in studies conducted by optometrists 5683-849198100-101 and by ophthalmologists 2-4 85 95

99102 Optometrists maintain that although vision may not be considered as a cause of dyslexia

compensating for visual defects make reading easier for the dyslexic child Ophthalmologists

hold the contrary view that vision does not in any way contribute to dyslexia Dyslexia has

also been thought to be a language-based learning disability80 Consequently from the speech-

language pathologistsrsquo perspective ldquoa child with a speech deficit may also have an emotionally

distorted perspective and a child whose classroom behaviour is hard to control may also be

unable to readrdquo The role of the psychologists is to perform psychometric assessments of the

dyslexic child32 The educator is primarily involved with special educational intervention for

the dyslexic child

Flax 22 proposed that the role of visual factors in dyslexia ldquodepends on the definition of vision

that is consideredPrime meaning that if vision is narrowly defined as visual acuity or clarity of

sight it is unlikely that there would be any relationship between vision and dyslexia If other

peripheral visual functions such as fusion convergence refractive errors and accommodation

are considered there is likely to be a relationship but then questioned the possibility of vision

being a causative factor Flax22 reiterated that the most important aspect of vision that is

closely linked to dyslexia is when vision is defined to include perceptual and integrative

aspects of vision It was concluded that ldquovision difficulties are part of the dyslexic

40

syndrome22

From a similar perspective Solan81 suggested that vision anomalies are contributory and

frequently hinder responses to educational intervention He emphasized that in cases where

vision anomalies are factors involved in the reading disability the effort required to maintain

clear single binocular vision decreases the efficient organization of the visual cognitive

response

This review has been structured in a way that an overview of a study will be made when it is

first cited Subsequently such studies will be cited only with references to the vision variable

being reviewed Only the studies that relate directly to the topic of the present research such as

visual acuity refractive errors near point of convergence ocular alignments accommodation

functions and vergence reserves are reviewed

It is also important to note that there are many variations in the results reported by different

authors In view of this the findings are thus presented on an individual basis Various studies

have attempted to study the relationship between dyslexia and vision and have examined the

different aspects of vision suspected of affecting reading ability However the major

drawbacks that plague the researches in the subject are Firstly intelligence quotient is a factor

to consider when defining dyslexia However only a few studies587-89 consulted have

considered intelligent quotient in their subjectsrsquo selection

Another area worth mentioning is the fact that some of the studies lacked control group which

made the reports difficult to assess and utilize In addition some researchers failed to classify

some visual functions such as binocular coordination and refractive errors thereby making it

difficult to evaluate the impact of a particular vision variable on reading ability Furthermore

some studies failed to indicate the eye examination techniques used in their protocol which

makes the replication of such studies difficultGiven the inherent limitations with studies

conducted to investigate vision in dyslexic children these studies are important in guiding the

teacher and clinicians on the appropriate intervention for the child

22 Visual Acuity

41

Vision plays an important role in the reading process as the ability to see printed material is

crucial in reading Reading efficiency and speed starts to decrease when the print size is less

than three times larger than an individualrsquos visual acuity threshold A marked bilateral

reduction in visual acuity can definitely limit reading performance but it is unknown if

deficient visual acuity characterizes students who have difficulty to read82

Available studies3-58384 that have attempted to see how visual acuity affects reading

performance are presented starting with the first three studies in this review that found a higher

prevalence of reduced visual acuity among dyslexics compared to normal readers

In 1994 Latvala Korhoenen and Penttinen 3 assessed the ophthalmic status of fifty five (55)

dyslexic and fifty (50) control subjects in an age gender and social class matched study in

Finland The subjectsrsquo school grade and intelligence quotient were not indicated Since

intelligence quotient is a major factor in the definition of dyslexia the lack of information on

intelligence quotient may lead to the questioning as to if the population studied were truly

dyslexics The authors assessed visual acuity using the Snellen chart and found that for the

control group all subjects had visual acuity of 07 (69) while in the dyslexic group two

children (36) had bilateral visual acuity of less than or equal to 07

A study to examine the lsquoophthalmologicalrsquo status of school children with dyslexia in Norway

was conducted by Aasved 4 in 1987 Visual examinations were performed on two hundred and

fifty nine dyslexic children The number of the participants from the control group was not

clearly stated The investigation techniques used was also not detailed As a result it may be

impossible for other researchers to replicate the study However Aasved 4 reported that the

dyslexic population had a higher prevalence of reduced visual acuity than the normal readers

did It was concluded that although there was no causal relationship between eye

characteristics and reading difficulties eye abnormalities should be corrected when detected in

dyslexic children

42

Evans et al 5 in 1993 studied some sensory and refractive visual factors in 39 dyslexic and 43

control subjects aged between seven years six months and twelve years three months in the

United Kingdom Participants from both groups had intelligence quotient of above 85 The

authors found that the dyslexic group had a significantly worse near and distance binocular

visual acuity the inter-ocular difference was not statistically different in the two groups and

the difference between the best monocular and the binocular visual acuity was not statistically

different between the two groups at distance or near They claimed that the ldquoreduced visual

acuity may not have been due to refractive defectsPrime Apart from refractive errors a reduced

visual acuity may arise from pathological conditions such as cataract and glaucoma

The following two studies reported similar prevalence of visual acuity worse than 69 In 1980

Hoffman83 investigated the incidence of vision difficulties in learning disabled children in the

United States of America Vision evaluations were performed on one hundred and seven

children with learning problems who were referred to the clinic for vision care from educators

and psychologists The subjectsrsquo mean age was eight years eight months As a comparison

(control) group twenty five patients between the ages of five and fourteen were selected from

the general optometry clinic The prevalence of visual acuity poorer than 69 was reported to

be 1028 among the learning disabled subjects The subjects from the control group were not

appropriately selected not representative enough and were not age-matched although the issue

of the comparison group being inappropriately selected was acknowledged by the author

Another limitation with the study was that no information was given on the eye examination

procedures followed This may be problematic as it makes comparison with other studies and

replication of the study difficult

Sherman84 conducted a study to ldquorelate vision disorders to learning disabilitiesrdquo in the United

States of America in 1973 Thirty-nine boys and eleven girls participated in the study The

children were diagnosed as having learning difficulties by the referring psychologists Their

43

ages ranged from six to thirteen years The technique used to measure visual acuity was not

indicated but it was reported that five children (10) had visual acuity poorer than 69

Other studies reported the prevalence of visual acuity to be similar between the control and

reading disabled group and include the following studies In 1985 Helveston Weber and

Miller85 (from ophthalmology) studied visual functions and academic performance in first

second and third grade children in the United States of America One thousand nine hundred

and ten children participated in this study The Snellen chart and the Jaegar print were used to

assess distance and near visual acuity respectively The authors reported that 94 of the

children tested had normal visual acuity and that visual acuity was not related to academic

performance in a positive way Although the study may be credited with a large sample size a

possible limitation with the application of the findings of this study was that the passfail

criteria were arbitrarily chosen by the examiners ldquoon the basis of previous clinical experiencerdquo

without references to other research studies Secondly the authors failed to present the results

of the statistical analyses of the relationships between vision functions and reading

Grisham and Simon82 conducted a review of literature on refractive error and reading process

and reported that there are abundant and reliable scientific evidence that shows a higher

prevalence of vision defects among reading disabled children than among normal readers It

was emphasized that there are cases where prescription of glasses has improved academic

performance Their analysis revealed no statistically significant difference in distance visual

acuity among normal and disabled readers but that near visual acuity may be an index which

distinguishes some children who are having difficulty to read and those who do not

It is important to note that since reading is a near point task a deficient near visual acuity may

be more contributory to poor reading than reduced distance acuity Consequently Grisham

and Simons82 stated that ldquodeficient distance visual acuity would not be expected to affect the

44

reading skills of children unless substantial reading instructions take place on the chalk board

It is possible that distance acuity is a factor in the early grades when reading is first

introducedrdquo

In 2007 Grisham Powers and Riles86 studied visual skills of 461 poor readers in California

high schools The participants mean age was 154 years and there was no control group in the

study Participants for the study had been identified by the schools as poor readers The vision

functions measured were visual acuity near point of convergence ldquoconvergence and

divergence and fusional rangesrdquo accommodative amplitude and accommodative facility No

information was given on IQ but it was mentioned that the basis for referral of students for

assessment was poor reading performance which was determined by the school and defined as

reading two grade levels or more below grade level Visual acuity was assessed using the

Snellenrsquos chart at six meters The authors reported that 568 of the participants had visual

acuity of 66 in both eyes 261 had visual acuity of 69 in each eye while 172 had visual

acuity of worse than 69 in either eye

Another comprehensive study conducted in 2007 was reported by Kapoula Bucci and

Jurion87 Convergence and divergence functions in 57 dyslexic and 46 non-dyslexic age-

matched children were studied in French dyslexic children The mean age of the dyslexic

subjects was 113 years the mean IQ was 105 and the mean reading age was 891 The mean

age for the control group was 107 years The IQ score of the control subjects was not

indicated The vision functions performed included visual acuity stereo acuity and cover test

near point of convergence and ldquomeasurement of convergence fusional amplitude (divergent or

convergent) Unfortunately the authors did not detail the testing procedures for the vision

functions investigated but reported the results of the study It was reported that all children had

normal visual acuity

In 2008 Alvarez and Puell 88 studied accommodation functions in school children with reading

difficulties in Madrid Spain Eighty seven poor readers (mean age 92 years) and 32 control

(mean age not indicated) participated in the study all in grades three through grade six

Relative accommodation amplitude of accommodation and accommodation facility was

assessed

45

The most current literature reviewed was reported by Bucci Gignac and Kapoula 89 in 2008

The authors studied poor binocular coordination of saccades in dyslexic children in France Of

particular interest to the current review is the assessment of visual acuity and refraction

(though was assessed under cycloplegia) heterophoria near point of convergence and fusional

vergence reserves The study participants comprised of 18 dyslexic children (mean age 114

years and mean IQ was 105) The control group comprised aged-matched non-dyslexic

children mean age 11 2 years There were no details on the method of assessment of visual

acuity and refraction but it was indicated that a cycloplegic refraction was conducted

However the authors reported that all children had normal visual acuity but the results for

refractive errors were not reported

In 1991 a study on stereopsis accommodative and vergence facility was conducted by

Buzzelli 6 in the United States of America Thirteen normal readers of average age thirteen

years three months and thirteen dyslexics average age of thirteen years four months

participated in the study Both groups were matched for gender age and intelligence quotient

The study design was comprehensive The Snellen chart was used to assess the visual acuity

Buzzelli6 found that the dyslexics did not perform worse than the control subjects in visual

acuity

Ygge Lennerstarnd Axelsson and Rydberg2 studied visual functions in a Swedish population

of dyslexic and normally reading children in 1993 Eighty six 9-year- old children matched to

controls with regard to age gender class in school and intelligence participated in the study

VA was assessed with ldquoordinary optotypes chartsrdquo They reported that the subjects from the

control group had a better visual acuity than the dyslexic group at both distance and near and

that the results showed statistically significant differences (distance at p =003) and (near at

p=0005)

A comparative study of dyslexic and normal readers using orthoptic assessment procedures

was conducted by Goulandris McIntyre Snowling Bethel and Lee90 The study was

conducted in 1998 in the United Kingdom and comprised 20 dyslexic and 20 chronological and

46

reading age-matched control participants A unique aspect of the study was that the distance

VA was assessed using the Cambridge Crowding Cards This test according to the authors

was devised to evaluate letter recognition acuity in conditions of lsquocrowdingrsquo when other letters

surround the target letter and that the test is designed to ensure that abnormalities of vision

which would not be revealed reliably using single targets were detected The near acuity was

assessed using the Snellenrsquos chart The use of different VA chart at distance and near may

raise a concern of standardization They reported that the prevalence of distance and near VA

(using a 69 criteria) was similar between the groups

In 1992 Evans Drasdo and Richards91 investigated optometric correlates of reading disability

in 10 children aged between eight and fifteen years who were referred to the optometry clinic

by the child psychologist The authors reported only the statistical values mean binocular

distance visual acuity of + 091 and mean binocular near visual acuity of + 089

47

23 Refractive Errors

In the study by Alvarez and Puell 88 no information was given on the protocol used to assess

refractive error but the refractive error results were reported For the poor readers mean

spherical equivalent refractive errors for the right and left eye were 020 06 and 020 06

respectively Fifty-seven (655) children were emmetropic In the control group mean

spherical equivalent refractive errors for the right and left eye were - 020 08 and - 014

080 respectively Twenty (645) children were emmetropic In contrast Evans et al 91

reported the mean spherical equivalent refractive error (by retinoscopy) of + 077DS for the

(right and left eye was similar)

231 Myopia

Myopia is characterized by a decrease in distance visual acuity Myopia would usually not be

expected to affect reading skills since students can make the necessary adjustments to their

reading positions However from a clinical standpoint unusually high myopia myopic

anisometropia and astigmatism could influence patients near performance and cause some

discomfort to the child There is no evidence in the literature that myopia is associated with

poor reading In the study by Alvarez and Puell88 57 of the children from the dyslexic

group were myopic compared to 194 from the control group

In 1987 Rosner and Rosner 93 conducted a retrospective comparison of visual characteristics of

data for two hundred and sixty one children with learning difficulties in the United States of

Ameria who had been identified by the schools as manifesting learning difficulties (LD) The

control group consisted of four hundred and ninety six children without learning difficulties

The subjects from both groups were of age range between six and twelve years The children

had been examined at the University of Houston Optometry Clinic The data collection

techniques used was not given but it may be assumed to be routine optometric eye examination

48

protocols because the study was conducted at the university optometry clinic The prevalence

of myopia (minus 050 dioptres) was reported to be 54 in the non-learning disabled patients

while 19 of the learning disabled patients were myopic

One limitation with this study however was that all subjects were patients of an eye clinic

where the two main reasons for referral are reduced visual acuity and school learning

problems Consequently the prevalence of visual defects in subjects selected from such a

clinical setting may have been higher than those from a non-clinical setting

In a review study conducted by Grisham and Simon82 the prevalence of myopia was lower in

the reading disabled group than among the normal readers Similarly Grosvenor 94 analysed

data from several studies and concluded that myopia is consistently associated with good

reading performance One limitation with this analysis however that is the conclusion drawn

from the review was based only on studies that found positive relationships between vision

anomalies and reading ability Grosvenor 94 however remarked that ldquomany authors failed to

find the relationship because they used ineffective methods

One of the earliest studies on the subject of vision and reading difficulty was conducted by

Eames 95 in 1948 in the United States of America He studied refractive and binocular vision

anomalies in one thousand (1000)rdquoreading failures one hundred and fifty (150) unselected

children and five hundred ophthalmic patientsrdquo Comparing the poor readers and the

unselected groups (control) he found the incidence of myopia to be same in both groups (4)

Myopia was defined as -100 diopter sphere There was no statistical analysis in that study and

the data collection techniques were vague although it was indicated that no cycloplegia was

used While the study may be criticized as not being current it is the major and early study to

give an insight into the subject of vision and reading disability

Eames95 and Shearer96 reported a similar prevalence of myopia (4) Shearer96 investigated

ldquoeye findings in children with reading difficultiesrdquo Two hundred and twenty two children

participated in the study The subjects were screened using the Keystone telebinoculars The

prevalence of myopia defined as - 050 dioptres sphere was 4 He concluded that vision

49

functions are not related to reading ability

232 Hyperopia

Hyperopia is often associated with poor reading performance probably through the mechanism

of deficient accommodation and poor motor fusion skills 82 A child with a relatively high

degree of farsightedness may not report blur vision at distance due to compensation for

hyperopia through the accommodation mechanism which also is used to create clear vision at

near This results in overuse of the accommodation system both optically and physiologically

and a subsequent fatigue or spasm of the accommodation system 97

One of the studies that classified refractive error in the literature was conducted by Rosner and

Rosner93 They reported that hyperopia appeared to be more prevalent (54) in learning

disabled than in the non-learning disabled children with a prevalence of 16

In the study conducted by Alvarez and Puell88 287 of the dyslexic group was hyperopic

compared to 161 from the control group Grisham and Simon82 in a literature analysis of

refractive errors and reading process reported that the prevalence of hyperopia among poor

readers is higher than among good readers and that the correction of hyperopia resulted in

improved performance

Grosvenor 94 reported that hyperopia is usually associated with poor reading Similarly in

Eamesrsquo study95 43 of poor readers had hyperopia while for the control group only 12 were

hyperopic The author gave no information on statistical analysis Shearer96 reported the

prevalence of hyperopia to be 16

233 Astigmatism

Clinically uncorrected moderate to high astigmatism can cause discomfort and lead to poor

reading performance at near distance Relatively few studies have attempted to study the

relationship between astigmatism and reading disability The prevalence of astigmatism

reported was higher in the dyslexic group compared to the control group in all the studies

reviewed 39395 These studies include the study by Latvala et al 3 who reported that 36 of

50

the dyslexic subjects were astigmatic (of greater than -100 diopter cylinder) and none from the

control group had astigmatism of that magnitude Eames95 reported the prevalence of

hyperopic astigmatism of more than -100 diopter cylinder among the poor readers to be 6

and 4 in the unselected group while the prevalence of myopic astigmatism was the same

(1) in both groups Rosner and Rosner 93 found a higher prevalence (30) of astigmatism

amongst the non-learning disabled subjects compared to the learning disabled group (27) In

Shearerrsquos 96 study of eye findings in children with reading difficulty the total prevalence of

astigmatism classified as myopic astigmatism (3) and hyperopic astigmatism (3) reported

was 6 There was no control group in this study In the study by Alvarez and Puell 88

astigmatism was detected in seven (8) and eight (92) right and left eyes respectively while

astigmatism was detected in five children (161) from the control group Ygge et al 2

reported that the prevalence of astigmatism was higher in the dyslexic group (28 left eye 25

) compared to the control groups (right eye 183 left eye 243) although there was no

statistically significant difference between the groups (p=025)

234 Amblyopia

Only two studies on the available literature consulted reported on the prevalence of amblyopia

both indicating a higher prevalence amongst the dyslexic group compared to the control group

In the study by Latvala et al 3 the prevalence of amblyopia in the dyslexic group was 36 and

none in the control group had amblyopia Although a detailed prevalence of refractive error

was not presented in that study it was reported that nearly all subjects had visual acuity of 07

In relating refractive error to amblyopia the prevalence of astigmatism in the control group

was 36 and none from the dyslexic group had astigmatism Consequently a low prevalence

of amblyopia may be related to fairly good visual acuities in both groups On the contrary

Rosner and Rosner 93 reported that 4 of the non-learning disabled patients had amblyopia

compared to the 3 found in the learning disabled patients Hyperopia is the refractive error

that is more often associated with amblyopia However this study presented a higher

prevalence of amblyopia (4) but a lower prevalence of hyperopia (16) in the non-learning

disabled patients However a difference of only 1 is not high enough to account for the

difference

51

235 Anisometropia

Anisometropia has been thought to be associated with poor reading skills probably through the

mechanism of poor sensory and motor fusion rather than reduced visual acuity It degrades

binocular coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 82

Three studies2395 reported findings on anisometropia Latvala et al 3 reported a higher

prevalence of anisometropia (of greater than or equal to one diopter) of 6 for the control

group than in the dyslexic group with 36 Similarly Ygge et al 2 found a higher prevalence

of anisometropia in the control group (158) than in the dyslexic group (94) Eames 95

reported that 13 of poor readers were anisometropic while 6 from the control group had

anisometropia According to Grisham and Simons 82 only few studies have considered

anisometropia to affect reading performance The authors concluded that there was insufficient

number of studies to draw firm conclusions from their review

Other studies2598 failed to classify refractive error according to the type These include

studies by Evans et al 5 who investigated some sensory and refractive factors in dyslexia The

distribution of refractive errors was found to be similar in both the dyslexic and control groups

The details of the participants profile was given earlier (section 21) The refractive errors

were assessed by distance retinoscopy and subjective refraction A cross cylinder was used to

measure astigmatism The spherical equivalent refraction was calculated by adding one-half

of the cylindrical components to the spherical component It was not indicated if refraction

was performed under cycloplegia Ygge et al 2 assessed refractive errors objectively using

streak retinoscopy under cycloplegia They reported that the distribution of refractive errors

were similar between the dyslexic and control groups

Sucher and Stewart 98 examined 136 fifth and sixth grade children comprising seventy two

with learning disability and 64 from the mainstream school as the control group The learning

52

disabled children were selected from a classroom of children attending special supervision and

instruction Children from both groups were matched by grade level No information on

intelligence quotient was given The testing protocol used to assess refractive error was not

indicated The authors reported a higher prevalence of refractive error in the dyslexic (208)

than in the control (94) group As indicated earlier refractive errors were not classified

according to the types in the three preceding studies 2598

Hoffman 83 reported the prevalence of refractive error to be 215 There was no report on the

control group and the techniques used to assess refractive errors were not indicated In the

study by Helveston et al 85 only 1 of the participants had refractive error when assessed

objectively with the retinoscope without the use of cycloplegia Refractive errors were not

classified according to the types (myopia hyperopia or astigmatism)

In 1991 Hall and Wick 99 conducted a study to investigate the relationship between ocular

functions and reading achievement in the United States of America One hundred and eleven

children (in grades one through grade six) participated in the study Eleven ocular functions

were investigated including visual acuity saccade eye movements accommodation facility

amplitude of accommodation accommodation posture heterophoria horizontal fixation

disparity vertical fixation disparity near point of convergence titmus and Randot stereopsis

Accommodation facility was measured using the flipper technique The amplitude of

accommodation was determined using the push up to blur technique Heterophoria fixation

disparity and near point of convergence were also measured but the techniques used were not

indicated The age range of the subjects studied was not indicated Using multivariate

correlation they found no statistically significant relationship between the ocular functions and

reading abilities as they noted that the acquisition of reading skills is given by many forces

which include the use of remedial instructions and language skills the role of peer pressure and

innate intelligence

53

24 Near Point of Convergence

Anomalies of convergence is one of the common problems encountered in the optometric

practice especially in the pediatric population and the reports on near point of convergence has

been presented comprehensively in the literatures reviewed

Grisham et al 86 measured the near point of convergence by asking the student if the clown face

target was seen as single when the examiner held it approximately 20 cm in front of the

studentrsquos eyes If so then the student was requested to say when the object appeared to double

as it moved closer to the face The examiner placed one end of an accommodative rule on the

studentrsquos forehead and judged where the two eyes diverged as the target moved toward the

studentrsquos face The measurement was repeated three times The authors found that 846 of

the students had near points at 8 cm or closer which they considered ldquonormalrdquo while 154

had near point of convergence of 9 cm or farther

In the study by Kapoula et al87 the NPC was determined by placing a small pen-light at

3040cm in the mid plane in front of the subject and moving it slowly towards the eyes until

one eye lost fixation The authors reported that the NPC was significantly more remote

(gt10cm) in dyslexics than in non-dyslexics (36 versus 15)

Bucci Gignac and Kapoula 89 followed similar protocol as in the study by conducted Kapoula

et al 87 The near point of convergence was determined by placing a small pen-light at 3040

cm in the mid plane in front of the subject and moving it slowly towards the eyes until one eye

lost fixation It was reported that the NPC was not statistically different between the two

populations the median value was 5 cm for both dyslexic and non-dyslexic children

Latvala et al3 failed to indicate the technique used in assessing the near point of convergence

54

but simply noted that ldquoconvergence ability was measuredrdquo but reported that convergence near

point greater than or equal to 8cm was found in 127 of the dyslexic subjects and 2 of the

control group The authors concluded that the near point of convergence was the only variable

that showed a statistically significant difference between the groups They concluded that

ophthalmic factors should not be overlooked as a contributing factor to dyslexia as they may

constitute part of the dyslexic syndrome and recommended that vision anomalies should be

corrected whenever detected

Shearerrsquos 96 findings are similar to the results reported by Latvala et al 3 He assessed the near

point of convergence using the keystone telebinoculars and reported that 12 of the

participants in his study had convergence problems

In a relatively elaborate study Evans Drasdo and Richards100 investigated accommodative and

binocular functions in dyslexia The subjects were 43 control and 39 dyslexics aged between

seven years six months and twelve years three months An intelligence quotient range of over

85 for the dyslexics and over 90 for the control with the Wechsler Intelligence Scale for

Children (Revised) (WISC-R) was recorded They measured the near point of convergence

using the royal air force (RAF) rule It was concluded that all subjects were able to report a

clear subjective break and recovery and reported no statistically significant difference between

the two groups in either the near point of convergence

Hoffman83 assessed convergence using keystone visual skills tests but grouped the results as

binocular coordination which makes it difficult to analyze and compare while in the study by

Helveston et al 85 98 of the participants had normal near point of convergence

In 2002 Keily Crewther and Crewther 12 investigated the relationship between functional

55

vision and learning to read in Australian school children Two hundred and eighty-four

children (mean age 99 18 years) received a vision screening emphasizing binocular

anomalies associated with discomfort at near (distance and near visual acuity distance vision

ldquochallenged with binocular +1 D lensesrdquo near heterophoria near point of convergence

stereopsis and accommodative facility) Children were classified as normal readers (n = 195)

children with dyslexia (n = 49) or learning disabled children (children who have learning

difficulty with impaired intelligence quotient) (n = 40) It was reported that the near point of

convergence were better in the dyslexic group compared to the normal and the learning

disabled group but found no statistically significant difference between the groups In

constrast Evans et al 91 assessed near point of convergence using the RAF rule and reported a

highly statistically significant relationship between NPC and reading retardation (p=0019)

25 Heterophoria

Heterophoria is a common binocular problem and in clinical practice it is particularly

important when analyzed in conjunction with fusional reserve as it gives an indication of the

zone of comfortable binocular vision

The distance phoria is a clinical measurement of the tendency of the two eyes to deviate from

parallelism when viewing a target usually at six meters When a closer viewing target is

utilized (say at about the individuals reading distance) the resulting near phoria measures the

degree to which the eyes over converges (esophoria) or underconverges (exophoria) to the

plane of the target97 This latent deviation requires fusional vergence to maintain single

binocular vision and at near they involve the accommodation system Smooth functioning of

these systems particularly at near is important for near tasks92

Several studies312858789959698100101 have attempted to investigate the role of heterophorias on

reading ability and all reported remarkably varying findings The assessment of heterophoria

was extensive in the study conducted by Latvala et al 3 Heterophoria at distance was evaluated

56

using the cover test and Maddox rod The prism bars was used to quantify their findings The

Maddox wing was used to assess phoria at near At near they reported a higher prevalence of

esophoria (61) among the control than the dyslexic group (58) a higher prevalence of

exophoria (25) among dyslexics than the control group (122) and a higher prevalence of

vertical phoria (61) among the control than the dyslexic subjects (36) At far distance

they found a higher incidence of esophoria (113) among dyslexics than the control (82)

subjects a higher prevalence of exophoria (2) among the control group than among the

dyslexics (19) and vertical phoria to be more prevalent in the control (41) than in the

dyslexics (38)

Keily et al 12 assessed heterophoria using the Prentice card and reported that there was a slight

but insignificant tendency for the dyslexic subjects to be more exophoric than the normal and

those with learning disability but found no statistically significant difference The test distance

was not indicated

Evans et al 100 in a study of accommodative and binocular functions in 43 control and 39

dyslexics reported that all subjects examined were orthophoric at distance with cover test

except for one dyslexic subject who demonstrated 10 prism diopters right esotropia and two

diopters right hypertropia which changed to 15 prism diopters right esotropia with two prism

dioptre right hypertropia at near At near horizontal phoria results were similar in both groups

Evans Efron and Hodge 101 studied the incidence of lateral phoria in 45 children with specific

learning disability in the United States of America in 1977 The participants were selected from

a psychologistrsquos file The control group comprised of 364 children in second third and fifth

grades attending regular school It was not indicated why there was such a big difference in

the number of participants from both groups The test used in assessing phoria was the

ldquoRocket card which was used in conjunction with red- green spectaclesrdquo They reported a

57

prevalence rate of 1135 in the control group while 222 of the children from the specific

learning difficulty group had heterophoria There was no statistically significant difference

between the groups In concluding it was recommended that phoria conditions may not cause

reading or learning disability but that the presence of such conditions could contribute to visual

perceptual and attention abnormalities

A report by Sucher and Stewart98 contained limited information as the authors did not detail

whether a higher prevalence rate among control (105) than dyslexic (86) was for near or

distance phoria Similarly Eames95 failed to indicate the distance at which the measurement

was made but reported that 33 of poor readers had exophoria and 22 of control were

exophoric but did not indicate whether distance or near

Helveston et al 85 used the Wottring amblyoscope to determine ldquoobjective and subjective angle

of ocular alignmentPrime and reported that 96 of the participants in their study were orthophoric85 The criteria used to classify ocular deviation were not indicated

Another comprehensive report of heterophoria was documented by Shearer96 The subjects

were screened using the Keystone telebinoculars and reported that 1 of the study participants

had vertical phoria (gt1 prism diopter) 2 had esophoria (gt 4 prism diopter) 3 had

exophoria (gt 4 prism diopter) 26 had exophoria at near It was concluded that vision

functions are not related to reading ability Hoffman 83 reported that ninety three (93) subjects

(869) had problems of binocular coordination but failed to indicate whether it was phoria or

tropia

Kapoula et al 87 evaluated ocular alignment using the cover test and reported that at far

distance the median value of phoria was zero for both non-dyslexic and dyslexic children (p =

019) At near the median value of phoria was similar between the two groups (minus2 prism

58

diopters for non-dyslexics and minus15 prism diopters for the dyslexics (p = 019)

Bucci et al89 measured heterophoria at near and far using the cover-uncover test As

documented by the authors exophoria was not statistically different between the two groups (p

= 02) the median was ldquo0rdquo prism diopters for dyslexics and 2 prism diopters for non-dyslexics

No information was given on other aspects of heterophoria

Simons and Grisham92 conducted a literature analysis on binocular vision anomalies and

reading emphasizing studies with positive and negative relationship between binocular vision

and reading The authors concluded that binocular conditions such as esophoria exophoria

restricted ranges of fusion foveal suppression accommodative insufficiency and oculo-motor

disorders at reading range are more prevalent among poor readers than normal readers

In another study Ygge Lennerstarnd Rydberg Wijecoon and Petterson102 investigated

oculomotor functions in a Swedish population of dyslexic and normally reading children in

1993 The study comprised 86 nine -year- old dyslexic children matched to control with regard

to age gender and intelligence Heterophoria was measured using the cover test They

reported that for the dyslexic group ldquo23 pupils exhibited a phoria (264 9 eso- and 14 exo-

phoria)rdquo whereas in the control group ldquo20 pupils had a phoria (232 6 eso- and 14 exo-

phoria)rdquo and that the difference was not statistically significant (p = 034) At near the

incidence of exophoria was 437 for the dyslexic group and 453 in the control group

There was no statistically significant difference (p=055)

26 Strabismus

Several authors34 83-85 90 93102 included the measurement of strabismus in their studies

reporting remarkably varying findings Helveston et al 85 assessed ocular alignment using the

Wottring amblyoscope and reported that only between 2 - 4 of the students had phoria or

59

tropia of more than four prism dioptres Latvala et al 3 reported the prevalence of tropia at near

for the dyslexic group to be 36 and 2 for the control group Hoffman83 assessed ocular

alignment using the keystone visual skills test and reported that 13 subjects (9 exotropes and 4

esotropes) (1214) had strabismus It is difficult to comment if this relatively high

prevalence of strabismus is related to hyperopia as the refractive error prevalence of 215 was

not categorized as to the type In a study by Sherman84 a total of four children (8) were

strabismic classified as three exotropia (alternating) and one esotropia (alternating) Rosner

and Rosner93 in their retrospective analysis reported that in the learning disabled group 51

had esotropia 39 had exotropia while in the non- learning disabled group 8 were

strabismic It was not unexpected to find a higher prevalence of strabismus in this

retrospective study Aasved 4 found that the dyslexics had a higher prevalence of manifest and

latent convergent strabismus than the control subjects

Goulandris et al90 assessed strabismus using the cover test and reported that there was no

statistically significant difference between the two groups in the prevalence of strabismus

Ygge et al 102 reported that at distance manifest strabismus was more frequent among the

dyslexic (8) pupils than the controls (23) but that the difference was not statistically

significant At near the prevalence of manifest strabismus was 8 (all eso-) for the dyslexics

and 35 (all eso-) in the control group

27 Accommodation Functions

271 Amplitude of Accommodation

The accommodation mechanism is extremely important for learning Children who suffer

some anomalies of accommodation are more prone to fatigue quickly and become inattentive

than those who have normal accommodation function According to Flax 22 an ldquoinefficient

accommodation function is a significant contributor to lowered achievement in the upper

grade

60

The findings on various aspects of accommodation functions were diverse Metsing and

Ferreira 103 studied visual deficiencies in children from mainstream and learning disabled

schools in Johannesburg South Africa in 2008 One hundred and twelve (112) children from

two learning disabled schools and eighty (80) children from a mainstream school in Johannes-

burg participated in the study The vision functions examined were visual acuity refractive

status ocular health status accommodative functions (Posture facility and amplitude)

binocularity (cover test vergence facility smooth vergences NPC and ocular motilities (direct

observation) However according to the authors only the results for accommodation

accuracy amplitude of accommodation vergence facility and saccadic accuracy are reported

since only these variables showed statistically significant relationship on the relationship

between the (mainstream and learning disabled) groups Metsing and Ferreira 103 reported

that a high percentage (516 and 533) of low amplitude of accommodation for the right

and left eyes respectively was found in the mainstream group compared to the 291 and

288 in the learning disabled group The relationships between the mainstream group and

reduced amplitudes of accommodation of the right (p=004) and left eyes (p=0001) were

found to be statistically significant (plt005) The relationship between the mainstream group

and reduced amplitude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to

be moderate and that between the mainstream group and the right eyes (Cramerrsquos V= 0286) to

be low

In the study by Grisham et al86 accommodative amplitude was assessed using the

accommodative rule ldquoThe student held an occluder over the left eye The examiner asked if

the student could see the (2030) target at the larger end of the Occlud-A Measure held at about

20 cm from the face If yes then the student was asked whether the target was in focus or ldquonot

blurryrdquo If the student responded that the target was in focus then the target was moved at

approximately 2cm per second toward the studentrsquos face and the student was asked to report

when the target first became blurryrdquo About 638 of the participants had amplitude of

accommodation of 11D which the authors considered equivalent to the expected amplitude of

61

accommodation for the age About 247 had inadequate accommodation amplitude while

115 had ldquoborderlinerdquo amplitude of accommodation

In the study by Evans et al 100 the amplitude of accommodation was measured using the push up

method They reported that the amplitude of accommodation was significantly reduced in the

dyslexic group and that the mean accommodation lag was calculated for each eye but found no

statistically significant difference between the two groups

Alvarez et al 88 assessed monocular accommodative amplitude using the minus lenses method

The subject viewed horizontal line of 2030 letters at 33cm as the examiner introduced minus

lenses in 025D increments until the target first becomes blurred According to the authors the

working distance adjustment used was kept at 250 DS to compensate for minification The

authors reported that monocular accommodative amplitude was significantly lower (p lt 0001)

in the group of poor readers (right eye 91 D 23 left eye 90 D 23) than in the control

group (right eye 105 D 17 left eye 105 D 17)

Helveston et al85 measured the amplitude of accommodation using the push up to blur

technique and reported that ldquoan ability to accommodate more than eleven dioptres was found in

between 90 and 93 of the studentsrdquo Ygge et al 102 assessed amplitude of accommodation

using the RAF rule and reported that the two groups performed similarly A similar technique

was used by Goulandris et al 90 and a lack of statistically significant difference between the

groups was reported

272 Accommodation Facility

In the same study Evans et al 100 reported that the dyslexic group appeared to be slower at a

62

test of accommodative facility The authors discredited the flipper technique as the

unnatural act of altering accommodation without changing vergence They further noted that

the flipper testhellip would still imperfectly reflect the normal visual environment owing to the

absence of a stimulus to vergence and accommodation

Grisham et al86 measured accommodative facility using the 200D lenses flipper technique

and the technique was performed as follows The 2D lenses was flipped each time the

student said the target was ldquoclearrdquo The target for this test was the 2030 target used for

accommodative amplitude The left eye was occluded The student was asked whether the

target was clear when held at about 40 cm If so then the student was told that the test would

be timed and that it should be reported whenever the target was ldquoclearrdquo The examiner then

started a stopwatch and counted the number of times the lenses were flipped in 30 seconds

That number was taken to represent the cycles per minute (cpm) and therefore to represent the

studentrsquos ability to change accommodation over time Based on the criteria used in their study

the authors reported that 76 of the students had adequate monocular accommodative facility

while 236 had ldquoweak accommodative facility of 9 cpm or less and 81 had accommodative

facility of as low as 1 or 0 cpm

In the study conducted by Alvarez et al 88 accommodative facility testing was conducted using

the following procedure The subject wearing his or her habitual correction was seated and

positioned so that the spectacle plane was 40cm from the Bernell Acuity Suppression Slide

(VO9) A 200 D accommodative demand was used with a 2030 test target at 40cm The

patient was asked to view the reduced Snellenrsquos letters 2030 in line 5 and say ldquonowrdquo when the

letters in that line appear clear and single and to report letters missing from lines 4 and 6

during the binocular testing They reported that the binocular accommodative facility values

were significantly lower (p lt 005) in the poor readers group (49 cpm 31) than the control

group (63 cpm 29)

63

Sucher and Stewart 98 assessed binocular accommodative facility at 35 cm using 150 diopter

flippers lens for ldquoone minuterdquo and ldquoused the University of California Berkeley clinic standard

of 20 cycles90s as a ldquocriterion for successrdquo found a higher prevalence of accommodative

infacility in dyslexics (222) than the control group (86)

On the contrary three authors6 12 93 found the control subjects to have better accommodation

facility than the dyslexic subjects The studies are Rosner and Rosner93 found a higher

prevalence of accommodative infacility among the control (10) than the learning disabled

group (9) Similarly Buzzelli6 found that the dyslexic subjects showed better

accommodation facility than the control group To assess accommodation facility ldquopatients

were asked to clear the number 9 on the Bernell Vectogramm Target at 40cm hellip alternately

viewed the target through a + 200DS and a ndash 200 DS lenshellip for 20 cycles) There was no

statistically significant relationship between the two groups

Keily et al12 assessed accommodative facility using +200DS and ndash 200DS flippers lenses

with an N6 fixation target for one minute at 30 cm rather than 40 cm because according to the

authors the children were required to hold the reading material They reported that

accommodative facility was better in normal readers than either of the poor reading groups

In another study Evans and Drasdo104 reviewed ophthalmic factors in dyslexia and commented

that convergence insufficiency accommodative dysfunctions and unstable eye movements are

more common in dyslexic population than in normal readers

Latvala3 simply noted that some aspects of accommodation were difficult to measure and failed

to give any details

64

273 Relative Accommodation

Alvarez and Puell88 measured relative accommodation using a phoropter The subject viewed a

horizontal line of 2030 letters at 40cm with hisher best refractive correction The examiner

introduced minus or plus spherical lenses in 025 D steps binocularly until the patient reported

first sustained blur Negative and positive relative accommodation values were similar in both

groups This was the only study accessed that reported on relative accommodation

274 Accommodation Posture

Evans et al100 assessed accommodation lag using the monocular estimate method of retinosocpy

and reported no statistically significant difference between the dyslexic and the control groups

Metsing and Ferreira103 measured the accuracy of the accommodative response (lag or lead)

objectively using the MEM retinoscopy in normal room illumination and found that the subjects

from the mainstream school demonstrated a high prevalence of lead of accommodation (40

and 405 for the right and left eyes respectively) However in the learning disabled group a

high prevalence of lag of accommodation (98 and 119 for the right and left eyes

respectively) was found The relationships were found to be statistically significant (p lt 005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000)

Sherman84 assessed accommodation amplitude and relative accommodation using minus lenses

and accommodative facility using the flipper techniques The lens powers for the plus and

minus lenses used to assess accommodation functions were not indicated His findings may be

difficult to utilize due to the somewhat unique way of classifying the results as ldquomechanical

problemsrdquo

Flax22 suggested further investigations in the nature of accommodative function in reading

65

disabled children indicating that the use of the push-up method of assessing the amplitude of

accommodation is unreliable due to its lack of sensitivity Flax22 reiterated that

accommodation has a contribution in learning disabilities and that accommodative function

inefficiencies tend to play an increasing role as the youngster moves through school and that

inefficient accommodation function is a significant contributor to lowered achievement in the

upper grade

28 Vergence Facility

Vergences are a clinical measure of an individualrsquos ability to overcome the prismatically

induced optical displacements of a target in space Convergence relates to each eye fixating

closer in space than the real physical location of the target while divergence is the measure of

the eyes to fixate further in space while still maintaining single binocular vision97

Buzzelli 6 assessed vergence facility as follows ldquopatients were asked to align a vertical picture

(box-X-O) slide made of anaglyph material for monocular presentation under binocular

conditions The target measured 21 mm in the vertical meridian and 10mm in the horizontal

meridian The subjects alternately viewed the target through 16 prism base out and 4 prism

base in The test was continued for 20 cyclesrdquo The author reported that the dyslexics

completed the vergence eye movement task significantly slower (300 seconds) than the normal

readers (240 seconds) A statistically significant relationship between the groups was reported

(p lt 005) It was recommended that the possible role of vergence in dyslexia should be

investigated on a larger sample of dyslexic and control groups

Metsing and Ferreira103 assessed vergence facility test at near (40 cm) with the subjects

presented with a vertical row of letters (69) using the 8 PD (BI) and 8 PD (BO) mounted in a

flipper device They reported that the prevalence of poor vergence facility (205) was found

to be higher in the mainstream group compared to the learning disabled group (183) The

relationship between the mainstream group and poor vergence facility (p = 0000) was found

66

to be statistically significant (plt005) A medium to strong relationship was found to exist

between the mainstream group and poor vergence facility (Cramerrsquos V = 0369) It was

concluded that children from mainstream schools are likely to present with poor vergence

facility compared to children from schools of the learning disabled

2 9 Fusional Reserves

The compensatory effect of fusional vergence reserves on phoria is very important as difficulty

with reading is likely to occur when there is uncompensated phoria in conjunction with

reduced fusional vergence reserves 92

A major study was reported by Evans et al 100 which found that negative and positive reserves

were significantly reduced in the dyslexic group relative to the control The ldquoprism vergence

amplitude calculated as the difference between base out and base in break and recovery points

was relatively reduced in the dyslexic grouprdquo They remarked however that several subjects

were unable to appreciate a subjective blur point but the results for the break and recovery

were complete

In the study conducted by Latvala et al 3 the incidence of fusional amplitude greater than or

equal to thirty two prism dioptre at near was higher in the dyslexic (75) than in the control

(61) group At distance they found fusional amplitude greater than or equal to15 prism

dioptre to be higher in the control (122) than in the dyslexic (94) group

Disordered vergence control was studied by Stein Riddell and Fowler 105 in the Uniteed

Kingdom in 1988 The subjects were thirty nine dyslexic children aged eight to eleven years

and a control group of twenty subjects of similar age group The vergence control was

measured using the Synoptophore They reported that two thirds of the dyslexic group had

67

abnormal vergence control According to the authors vergence eye movements never seem to

have been investigated in detail during reading before and that the study was the first in which

eye movement recording have been used to demonstrate differences between dyslexics and

normal readers

According to Grisham et al86 ldquoConvergence and divergence break and recoveryrdquo were

measured at 40 cm with a horizontal prism bar The student was first asked if the clown face

target on the smaller end of a standard optometric measure (Occlud-A-Measure Bernell

Mishawaka Indiana) could be detected If so then the student was requested to say whether

the target appeared single or double Once single vision was obtained the bar was moved

relative to the studentrsquos eye until double images were reported That point was considered

ldquobreakrdquo Then the motion of the bar was reversed until the student reported the image single

and this point was considered ldquorecoveryrdquo This procedure was first performed for base in (BI)

position of the bar over the left eye and repeated with base out (BO) position over the right

eye The authors reported that a large number of students had poor convergence skills 38

break at less than 18 PD and 95 recover at 7 PD or less These values were considered to be

in the ldquoweakrdquo to ldquovery weakrdquo range and remarked that in contrast to convergence lsquomodalrdquo

values for break (14 PD) and recovery (10 PD) were not in the ldquoadequaterdquo range but would be

classified as ldquovery weakrdquo for break and ldquoweakrdquo for recovery As a result very high

percentages of students had poor divergence skills 82 break at less than 20 PD and 60

recover at less than 11 PD

Kapoula et al 87 measured vergence reserves using prism bars at far and near distance for both

groups They reported that the median value for positive fusional vergence was 16 prism

diopters and 14 prism diopters for non-dyslexics and dyslexics respectively at far distance (p

= 012) and 20 prism diopters for both non-dyslexics and dyslexics at near distance For

negative fusional vergence they reported that ldquodivergence amplituderdquo was significantly

different in the two groups for both distances (p lt 0005) At far the median value was 6

68

prism diopters and 4prism diopters in non-dyslexics and dyslexics respectively and 12 prism

diopters and 10 prism diopters respectively at near

Bucci et al 89 measured fusional reserves at far and near using prism bars (base in and base

out) According to the authors orthoptic evaluation of vergence fusion capability showed a

significantly limited divergence capability for dyslexics compared to non-dyslexics the

median value was 10 prism diopters for dyslexics versus 17 prism diopters for non-dyslexics

In contrast convergence amplitude was in the normal range for the two populations while for

dyslexics the amplitude was significantly larger than for non-dyslexics the median value was

30 prism diopters and 18 prism diopters for dyslexic and non-dyslexic children respectively

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups

The mean fusional convergence capacities at distance were 1680 prism diopters for both the

control and the dyslexic group At near the corresponding figures were 2640 prism diopters

and 267 prism diopters respectively The mean fusional divergence capacity at distance was

650 and 620 prism dopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 105 and 102 prism diopters respectively There was no

statistically significant difference between the groups

210 Ocular Pathology

Ocular pathologies do not seem to be prevalent in the dyslexic population and it was not

reported on in the literature reviewed However according to Evans and Drasdo100 the

perspective to relate ocular pathology to dyslexia is in individuals who were normal readers but

developed reading disability due to pathological conditions such as stroke or tumors affecting

the right hemisphere These conditions may be relevant to the considerations of establishing the

relationship between ophthalmic problems and dyslexia100

69

Only one study83 included this as part of the investigation Hoffman83 reported that one child

(093) of the population of learning disabled children examined had acute conjunctivitis

A major study on dyslexic subjects (although outside the cope of the present study but worth

citing) was conducted by Raju11 in 1997 She studied oculomotor control in dyslexia and

reported that sixty five percent of the normal readers exhibited normal oculomotor control as

compared to the eleven percent of dyslexics and that fifty four percent of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills whilst twelve

percent of the control subjects displayed the same

70

LITERATURE REVIEW TABLE pages 70-72

71

72

73

211 Summary

Research into visual aspects of dyslexia has been undertaken mainly by optometrists and

ophthalmologists The main area of controversy is that ophthalmologists tend to deny any

relationship between vision and reading ability while optometrists assert that identifying and

correcting visual defects are an important part of managing dyslexia The findings of studies that

have investigated vision function in dyslexic population since the 1940s are inconclusive and

reflect the complexities associated with identifying the causes of dyslexic as well as its diagnosis

treatment and management

A range of study designs were used and the inclusion of a control group for example depended

on the objective of the study Some authors either compared the prevalence of vision defects in a

population of reading disabled group to the prevalence of similar vision defects in a group of

normal readers or only presented the prevalence of a particular visual condition in a group of

reading disabled children All the studies used convenience sampling methods at schools or by

including learners who were referred to their private practices the size being determined by the

number of dyslexic students available The main areas of vision functions studied were visual

acuity refraction binocular vision and ocular pathology The study findings were inconclusive

and the only vision variables consistently report ed across the studies was the association of

hyperopia with poor reading performance

The differences in results reported by the authors may be due to the methodological problems

such as differences in instrumentation and techniques failurecut-off criteria method of data

analysis lack of comparison group and subjectsrsquo selection from clinical population There was a

noticeable absence of studies on African children research has shown that African persons are

less at risk of myopia and more at risk for open-angle glaucoma than Caucasians There is

therefore scope for continued research among African children to determine whether there is any

relationship between dyslexia and vision

Research on dyslexia and vision that requires an interdisciplinary approach may yield different

results due to different approaches by different professionals However it is challenging to

74

design a study that would control for all variables possibly influencing reading ability and vision

function Reading performance is an extremely complex task and its measurement can be

influenced by many factors

Having reviewed the relevant literature on dyslexia and vision Chapter three will present the

research design the study population sampling procedure methods of data collection and

analysis

75

CHAPTER THREE METHODOLOGY

31 Introduction

This chapter will present the research design the study population sampling procedure

methods of data collection as well as method of data analysis

32 Research Design

The study was designed to provide empirical information to enable a comparison of the visual

characteristics of dyslexic children and normal readers A matched paired case-control

method was adopted Data was collected from both the dyslexic and control groups and

analysed

321 Sampling Design

Participants for both samples were selected using the convenient sampling method (based on

available subjects) and an optometric eye examination was conducted

322 Study Population and Participants

The study population comprised children attending a school for children with learning

difficulties from which the dyslexic children were selected to participate in the study while

the control group consisted of learners from a mainstream Durban school

i Dyslexic Group Participants for the study from the dyslexic group consisted of 31

African dyslexic children (the experimental group) selected from Khulangolwazi Special

School for children with learning difficulties in Clairwood South of Durban Psycho-

educational evaluationdiagnosis of dyslexia was not part of this study and was not

76

performed The school principal allowed access to all learnersrsquo file from which the

dyslexic learners where selected based on the school psychologistrsquos diagnosis Learners

attending this special school were referred from mainsream schools around Durban The

final diagnosis as to the type of learning difficulties was done by the school psychologist

None of the children were on any medication

The mean age for the dyslexic participants was 13 years and the learners were in grades

four through grade seven considering the fact that they were children that were reading two

grades behind their chronological age There were 15 boys and 16 girls Due to limited

number of learners who were classified as dyslexic it was difficult to recruit the targeted

number of one hundred participants for the study The subjects from both groups were

lsquoAfricanrsquo South Africans of African origin

ii Control Group Participants for the control group consisted of 31 children from a

mainstream school in Durban (Addington Primary School) The mean age of the children

was 11 years and 9 months A total of 15 boys and 16 girls were chosen ranging from grade

four to grade seven

323 Inclusion Criteria for the Dyslexic group

To be included in the study the subjects had to meet the following criteria

1 An average or above average intelligence quotient For this study an average was taken

as between 95 and above as recorded on the learners file by psychologist

2 Be two grades or more below grade equivalent in a mainstream school

3 The child has not been absent from school for more than 10 of the attendance days

The information on the learnersrsquo attendance was supplied by the school principal

324 Exclusion Criteria for the Dyslexic group

The following criteria resulted in children being excluded from the study

1 Presence of any emotional problems (information from the learnersrsquo file)

2 Presence of any systemic condition

77

3 Use of any systemic medication

The inclusion and exclusion criteria for the control group were similar to the experimental

group except that the children did not have any reading problems and were attending a

mainstream school

33 Ethical Clearance

The study protocol was approved by the then University of Durban-Westville research ethics

committee A letter of request for access to the schools was sent to the Department of

Education (Appendix B) Written informed consent (Appendix C) for access to the schools (for

both groups) was obtained from the Department of Education Written informed consents were

also obtained from the principals of the schools (mainstream and special school) (Appendix D)

Due to the difficulty in reaching the parents the school principals consented on behalf of the

parents for the learners to participate in the study The participants were fully informed of the

purpose of the study and accepted to co-operate with eye examination procedures

34 Testing Protocol

The testing was done first with the dyslexic learners and then at the mainstream school

35 Study Setting

The same testing arrangement was used for both groups A testing room was set up in one of

the offices provided by the school principal The distance acuity chart was placed at a point

six meters away from where the subjects sat and was used for the distance acuity chart and all

distance testing The examinations were done under room illumination except for some

procedures such as retinoscopy that required dim illumination resulting in the room being

dimmed

78

All testing was conducted in the morning as it was anticipated that better responses could be

obtained when the children were not tired Each school principal provided an assistant who

helped with various activities such as controlling room illumination The rationale and

technique for every procedure was fully explained to each participant and a trial reading was

taken to ensure that all instructions given were understood During the tests subjects were

asked (from time to time) if they were tired For any answer to the affirmative the testing was

discontinued and the child was allowed to have a break As a routine a break of 10 minutes

throughout the duration of entire procedure was allowed

The testing instruments were provided by the International Center for Eye Care Education

(ICEE) As the instruments were also being used by other researchers they were unavailable

on certain days resulting in some tests being postponed until a later date The data collection

took an average of two months per school All data were collected by the examiner alone and

each examination took an average of thirty minutes to complete

36 Data Collection Techniques

All tests were conducted in free space The data collection techniques used in the present

study was similar to techniques used in other studies 93100106-140 based on pediatric

populations (Table 31)

Table 31 Data Collection Tools and Measurement Outcome

InstrumentationTechnique Measurement Outcome

Bailey ndashLovie LogMAR Charts Visual Acuity

Royal Air Force (RAF) Rule NPC Amp of Accommodation

Welch Allyn Streak Retinoscope Objective Refraction

Trial Frame and Trial lenses Subjective Refraction

Cover Test - Occluder and Prism Bar Ocular Alignment

Maddox Rodpen torchPrism bar Heterophoria and Tropia

Maddox Wing Near Phoria

Prism Bar Heterophoria and Tropia

79

Pen torch PD rule Cornea reflex test-strabismus

Retinoscope and neutralizing lenses Accommodation Posture

Jackson cross cylinder Refine subjective findings

Trial Frame and neutralizing lenses Relative accommodation

+-2 flipper lenses Accommodation Facility

Direct Ophthalmoscope (Welch Allyn) Ocular Health

Prism Bars Fusional Reserves

361 Visual Acuity

Visual acuity is a measure of the acuteness or clearness of vision It assesses the ability of the

visual system to resolve detail which is dependent on the sharpness of the retinal focus within the

eye the sensitivity of the nervous elements and the interpretative faculty of the brain 106 Visual

acuity was assessed using the Logrithm of Minimun Angle of Resolution (LogMar) chart which

means that the letters change in size from line to line in equal steps of the log of the minimum

angle of resolution (MAR) Visual acuity was assessed monocularly and binocularly for both

distance and near The LogMar chart facilitates algebraic operations for statistical analysis and

allows for precise quantitative assessment of visual acuity It is recommended for use in research

studies in which visual acuity is a dependent variable106

Each subject was comfortably seated and the test was conducted with the examiner sitting in front

in such a way that the subjectrsquos view was not obstructed For monocular visual acuity the

subject was instructed to cover one eye with their right palm and to read the letters on the chart

This procedure was repeated binocularly and the test was done under normal room illumination 106 No subjects wore spectacles so visual acuity for aided vision was not indicated

362 Refractive Error

Refractive error was assessed objectively using the Streak Retinoscope (Welch Allyn) with a

+150D fogging lens (for an armrsquos length of approximately 67cm) with the subject fixating a

660 (to maintain fixation) optotype on the distance visual acuity chart As the subject focused

80

on the distance target on the chart the retinoscope light was passed across the subjects eye

(right hand used to scope right eye and left hand used to scope left eye) The motions

observed were neutralized with the appropriate lenses plus lenses for lsquowith motion and minus

lenses for against motion The subjective refraction was performed through monocular

fogging refined using the Jacksonrsquos cross cylinder followed by binocular balancing 106 The

retinoscopic results were used for analysis

Although retinoscopy was performed without cycloplegia as in other studies 395107108

cycloplegic refraction was not performed because the researcher (an optometrist) was not

licensed to use therapeutic drugs when the data was collected However it has to be

acknowledged that a non-cycloplegic examination among children has limitations among

those with hyperopia as the full extent of the hyperopia may be masked

363 Near Point of Convergence

The NPC was measured using the Royal Air force (RAF) rule The subject was instructed as

follows ldquoThis test measures your ability to turn your eyes in towards your nose Look

directly at the dot on middle of the line The image may appear to be blurring (not clear)

That is okay However if the dot becomes two say lsquotworsquo I will then pull the target away If

the dot becomes one again say lsquoonersquo rdquo The subject was asked to fixate on the dot on the

middle of the line as the target was advanced towards him The objective reading was taken at

the point when one eye loses fixation while the subjective reading was noted when the subject

reported the target to be double At a point when the subject reported double the target was

moved back until a point when the target was reported to be single again this was taken as the

recovery point The final break and recovery points recorded were the average of three tests

measurements The measurement was taken three times in order to detect fatigue which may

indicate poor convergence 111-113 The objective reading was taken for analysis

364 Strabismus

Strabismus was assessed using the Hirschberg test A penlight was held about 50cm from the

childs face in the mid plane and they were encouraged to fixate the light with both eyes open

81

The location of the corneal reflection on each cornea relative to the centre of the pupil was

then noted The displacement of the corneal reflection from the centre of the pupil (angle

lambda) was estimated in millimeters An ocular alignment was observed as a symmetric

displacement of the reflexes 114115 The examiner proceeded with the Kappa Test which is a

test of monocular fixation and is done while the child was monocularly viewing the examiners

penlight The penlight was maintained at 50 cm distance from the face The purpose of this

test was to determine the visual axis of each eye The examiner observed the location of the

corneal reflexes relative to the center of the pupil in the right eye and then in the left eye A

nasal displacement is signified by a (+) sign and a temporal displacement is signified by a (-)

sign with the amount quantified in millimeters away from center Again a normal location

will likely be either centered or +05 mm nasally displaced 114115

The interpretation of the magnitude and the type of deviation requires the data gathered from

both the Hirschberg tests The fixating eye is the eye in which the corneal reflex location is

the same on the Hirschberg and Kappa tests The strabismic eye is the eye in which there was

a difference The change in location from the Kappa tests as compared to the Hirschberg test

equals the amount and type of deviation Every 10 mm of corneal displacement is

approximately equivalent to 22 prism diopters A nasal change in location on Hirschberg

testing from the visual axis denoted by the Kappa test denotes an exotropia A temporal

change in location on Hirschberg testing from the visual axis denoted by the Kappa test

denotes an esotropia 114

365 Heterophoria

Ocular alignment was assessed using the cover test at six meters and 40cm for distance and

near respectively It is an objective technique and is very suitable for assessing ocular

alignment in children

For distance cover test the test target was a letter from the line above the subjectrsquos best visual

acuity of the worst eye The subject was advised that the muscle balance of their eyes was

assessed and that he should look steadily at the fixation target (an isolated letter on the chart)

at all times and ignore the occluder The coveruncover (unilateral) cover test was performed

82

to determine the presence of phoria or tropia 106114-115 To perform the unilateral cover test

the occluder was placed in front of the right eye held for approximately one to two seconds

and the left eye was observed for any movement If no tropia or phoria present the left eye

was covered and the right eye then observed Any movement observed was neutralized using

a loose prism bar in the corresponding base direction Base-in prism for exo deviation and

base-out for eso deviation

The alternating cover test was performed to assess the direction and magnitude of phoria or

tropia (it measures the total angle of deviation) 106114-115 The test conditions were the same as

the unilateral cover test The subject was comfortably seated and instructed to look at the

fixation target at distance The occluder was placed in front of the patientrsquos right eye held for

2-3 seconds and then quickly placed on the left eye The test was repeated several times with

the occluder held in front of each eye before quickly moving to the other eye while observing

the eye that was just uncovered Any deviation was neutralized accordingly 115 The Maddox

Wing was also used to assess near phoria under normal room illumination The technique was

assessed with the subject wearing his full subjective correction 115 The use of cover test for

the assessment of ocular alignment at both distance and near may have allowed for continuity

and standardization However the cover test and the Maddox wing were used by Evans et al 100 in the same study The subject was directed to look through the horizontal slits to view the

chart that comprised of a horizontal and vertical scale and a horizontal and vertical arrow The

construct of the Maddox wing is such that the right eye sees only the arrows while the left eye

sees only the scale It achieves dissociation by presenting independent objects to the patient

As the images are dissimilar the incentive to fusion is abolished and the eyes adopt the fusion

free position 115 The arrows are positioned at zero on the scales but through dissociation any

phoria will be indicated by an apparent movement of the arrow along the scale

To measure horizontal phoria the subject was asked ldquowhich white number does the white

arrow point tordquo The number on the scale represents the magnitude of the deviation and the

direction To measure vertical phoria the subject was asked to indicate which red number the

red arrow points to 115

83

366 Evaluation of Accommodation Functions

a Accommodation Posture The assessment of accommodation accuracy was performed at

the subjects habitual near distance using the monocular estimation method (MEM) The

MEM technique uses a specially made target with pictures or words suitable to the childs

reading level The card was attached to the retinoscope The procedure was performed

over subjects distance prescription and at his normal reading distance with enough light

to provide a comfortable reading vision Retinoscopy was performed on axis (right eye

before left eye) as the child reads the words or describes the picture on the card aloud As

the retinoscopic reflexes were observed an estimate of the magnitude of the motion was

made and trial lenses were then interposed briefly (approximately 05 seconds) to avoid

changing the childs accommodation status until no motion was observed The lens

power at which the motion was neutralized was taken as the lag or lead of

accommodation It is a lag of accommodation when the reflex is neutralized with

positive lenses while it is called lead of accommodation if neutralized with negative

lenses 112113116117

b Accommodation Facility this assesses the rate at which accommodation can be

stimulated and relaxed repeatedly during a specific time period It relates to the

individualrsquos ability to shift focus quickly and efficiently for varying distance The

procedure was performed with the best subjective correction in place monocularly and

binocularly but only the result for the binocular assessment was utilized for the data

analysis According to Wick and Hall 120 binocular accommodative facility testing may

not be a true accommodative facility measure such as monocular facility but clinically

binocular testing may be more useful because it gives a ldquoreal liferdquo assessment of

accommodation and binocular interactions of accommodation and vergence

The target used was letters on a 69 range on a near point card and at the subjectrsquos

habitual reading distance A lens flipper of 2 diopter lenses was used Letters and

pictures used were appropriate to the childs ability The subject was informed that he

would be shown two different lenses and that one pair causes the system to work while

84

the other pair relaxes it and was instructed to look at the materials as the lenses were

flipped Each time the print become clear and single he was to immediately report

clear The lenses were flipped and child should report clear when the material gets

clear To ensure that subject understood the instructions trial readings were first taken

The number of cycles completed per minute was recorded One cycle meaning clearing

both the plus and minus lens sides The test was done binocularly112-113118-120 At near

with no suppression control

c Amplitude of Accommodation The amplitude of accommodation was assessed by

Donderrsquos push ndash up method using a Royal Airforce (RAF) near point rule a rod with a

movable target and metrics as well as dioptric marking This procedure was performed

monocular and binocularly The subject was seated comfortably and instructed to read a

line of letters on the card that corresponded to a visual acuity of 69 and told to keep the

letters clear The target was slowly moved towards the child along the rule until the child

reported first sustained blur at which point the dioptric result was read off the Royal air

force rule Three readings were performed and an average taken This test was

conducted monocularly for each eye and then binocularly

As reported by Wick and Hall 120 the minimum amplitude of accommodation expected is

based on the subjects age and can be estimated from the Hofsetterrsquos formula

Minimum =15 - 025 (age)

Expected =185 - 03 (age)

Maximum = 25 - 04 (age)

The amplitude of accommodation was generally considered clinically important only

when it falls below the expected age minimum 120

d Relative Accommodation (RA) Relative accommodation tests are plus-to-blur (negative

relative accommodation) (NRA) and the minus-to-blur (positive relative accommodation)

(PRA) The relative accommodation tests assess patientsrsquo ability to increase and decrease

accommodation under binocular conditions when the total convergence demand is constant 106 It is also an indirect assessment of the vergence system since the vergence demand

85

remains constant while accommodative demand varies 121122

Relative accommodation was measured using the plus and minus lenses binocularly at

near with distance subjective results in place 122 In this procedure the child viewed a

reading target size letter size or picture of 66 size at the reading distance with the best

subjective correction in place Plus or minus lense powers was added in 025 diopter

steps in approximately 2 seconds interval121 to assess negative and positive

accommodation respectively This test was performed binocularly and the reading was

taken when the subject reported a sustained blur The relative accommodation is difficult

to assess with trial lenses121 The trial lenses were used to assess relative accommodation

in the present study The use of the phoropter was not possible considering the study

setting

367 Fusional Reserves

The term fusional reserve is used synonymously with vergence reserves and prism vergence

vergence amplitude 121123 or fusional amplitudes 111 Vergence ability is measured using

prisms placed in front of each eye The amount of prism is increased gradually to measure the

amount of fusional reserve the individual has in reserve to compensate for a phoria112 124 The

amount of base out prism required to produce diplopia is called positive fusional reserves or

positive fusional vergence while the amount of base in prism required to produce diplopia is

called the negative fusional reserves or negative fusional vergence 115

Positive fusional vergence (convergence) and negative fusional vergence (divergence) were

assessed using a prism bar in steps without suppression control although it has been reported

by Wesson et al 125 that when suppression is controlled the average vergence values will be

lower because the test is stopped when the suppression is detected that is if suppression is not

monitored the break is not detected until the stimulus is outside the suppression zone and a

higher vergence value may be obtained However

Scheiman et al 126 argued that the significance of such suppression in binocular individuals is

unknown

86

Assessing fusional reserves in children with loose prism bars may be a better technique

because it offers the possibility of viewing the eye movement objectively Secondly it

removes the restriction imposed by testing children behind the phoropter Finally the test

itself nearly duplicates the movement of the eyes under normal conditions allowing

convergence or divergence in discrete steps rather than as a smooth-pursuit type vergence

movement 125

The test was performed with the target at six meters (test target 69 letter line) and at 40 cm

(N5 text) The prism was placed in front of the right eye first Base-in was routinely assessed

first the rational for this sequence of testing being that the convergence response stimulated

during the base-out (or convergence) measurements may produce vergence adaptation ( a

fusional after-effect) which may temporarily bias the subsequent base in values in the base-out

direction 127 Typically there is no blur point for base -in vergence testing at distance This is

because at distance accommodation is already at a minimum and cannot relax beyond this

point 115

Base out prisms were used to measure positive fusional vergence while base in prisms were

used to measure negative vergence The subject was seated comfortably and instructed as

follows ldquoI am measuring the ability of your ayes to converge and diverge Watch the row of

letters Tell me when they first get blurry if they do Also tell me when they first become

two rows of letters and when they become one againPrime The prism was introduced over the

right eye lsquoDoublersquo was first demonstrated to the child by placing a 20 diopter prism over one

eye If blur was reported the prism power was increased until a break was reported The

prism power was then reduced until the subject reported that the rows of letters were single

The technique was then repeated for base out prism and then the test was repeated at near The

break and recovery points were determined subjectively from the childrsquos report of blur break

and recovery and objectively by observing the subjects eye movements The objective

findings were used for analysis

87

368 Ocular Health

The only test performed to evaluate ocular health was direct ophthalmoscopy

369 Vergence Facility

It was impossible to perform the vergence facility testing as the children could not fuse the

base in prisms So vergence facility data was not available for analysis The recovery point

of the base in reserves may provide useful information on the vergence facility According to

Bishop 111 ldquoblur and break points tend to reflect the quantity of fusion whereas the recovery

point indicates the quality of fusion that is the ease of change of fusional demands (facility)

and the ability to maintain fusion (stamina)

37 Statistical Analysis

The data entry was done by a staff optometrist who is skilled in statistical analysis The data

analysis was undertaken by the International Center for Eye Care Education statistician Data

was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 For comparison all data from both groups was subjected to a two-sample t-test (2-

tailed) (Table 32) Participants who did not meet the pass criteria for any of the test variables

were referred to their optometrist for further evaluation

Table 32 The Diagnostic Criteria for Each Test Variable

Variable Diagnostic Criteria

Visual Acuity 128-131 69

88

Refractive Errors 128-131

Hyperopia ge +075

Myopia ge -050

Astigmatism ge -075

Anisometropia ge 075 between the two eyes

Emmetropia lt - 050DS lt +100DS lt -075DCyl

Near Point of Convergence 132-134 ge 10cm

Accuracy of Accommodation 116117119120

Lag ge +075 Lead any minus finding

Accommodative Infacility 118119 gt 7cycles per minute in response to +2 -200 flipper testing

binocularly at near

Amplitude of Accommodation106119120

Binocular accommodative amplitude ge 2D below the expected

value for the patients age for minimum amplitude (using

Hofsetterrsquos formula) (15-14age)

Positive Relative Accommodation 106121122124

gt - 237DS

Negative Relative Accommodation 106121122124

gt + 2 DS

Phoria 106128

Distance

Near

gt 6 prism diopters exophoria or 4 prism diopters esophoria

gt 6 prism diopters exophoria or 4 prism diopters esophoria

Strabismus 131135 Manifest or intermittent manifest deviation of gt 2 prism

dioptres 135 or asymmetry in Purkinje reflex or compensating

fixation a misalignment of ge 25 degrees 131

Fusional Reserves LimitsDistance 119

Near 106121126

Base in Blur X Break 6 Recovery 4

Base out Blur 10 Break 16 Recovery 10

Base in Blur 14 4 Break12 5 Recovery 7 4

Base out Blur 22 8 Break 23 8 Recovery 16 6

89

38 SUMMARY

The study consisted of two groups of 31 participants from two schools in Durban The study

group was children with learning difficulties from which the dyslexic children were selected

and the control group was selected from a mainstream school The mean age for the dyslexic

participants were 13 years while the mean age of the children from the control group was 11

years and 9 months A convenience sampling method was adopted to select the study

participants due to the limited number of subjects to select from

The vision functions investigated may be divided into three broad areas of visual acuity and

refraction binocular vision and ocular pathology which corresponds with areas of vision

function investigated in the literature review Standard optometric testing procedures were

used to assess the vision variables

Visual acuity was assessed using the LogMAR chart at both distance and near Refractive

error was determined objectively using a streak retinoscope (without cycloplegia) while the

child fixated a 660 optotype at 6 meters with a fogging lens to control accommodation

The near point of convergence was measured using the Royal Air force rule (RAF) Ocular

alignment was assessed using the cover test at six meters and 40cm for distance and near

respectively For distance cover test the test target was a letter from the line above the

subjectrsquos best visual acuity of the worse eye The coveruncover (unilateral) cover test was

performed to determine the presence of a phoria or tropia The alternating cover test was

performed to assess the direction and magnitude of the phoria or tropia The test conditions

were the same as the unilateral cover test The Maddox Wing was also used to assess near

phoria under normal room illumination Strabismus was assessed using the Hirschberg test

with a penlight held about 50cm from the childs face in the mid plane and was encouraged to

fixate the light with both eyes open The location of the corneal reflection on each cornea

relative to the centre of the pupil was then noted

The different aspects of accommodation functions were also measured The assessment of

accommodation accuracy was performed at the subjects habitual near distance using the

90

monocular estimation method (MEM) The accommodation facility was assessed using a 2

diopter lens flipper with the best subjective correction in place The target used was letters on

a 69 range on a near point card and at the subjectrsquos habitual reading distance The amplitude

of accommodation was assessed by Donderrsquos push ndash up method using a Royal Airforce (RAF)

near point rule This procedure was performed monocular and binocularly with a 69 visual

acuity as the test target The relative accommodation was measured using the plus and minus

lenses The test target is a reading letter target size or picture of 66 size at the reading distance

with best the subjective correction in place This test was performed binocularly and the

reading was taken when the subject reported a sustained blur

The fusional reserves were assessed using prism bars in steps without suppression control

The test was performed at 6 meters (test target 69 letter line) and at 40 cm (N5 text) Base out

prisms were used to measure positive fusional vergence while base in prisms were used to

measure negative vergence Ocular health evaluation was assessed using the direct

ophthalmoscope

Data was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 To compare the data from both groups all data was subjected to a two sample t-test

(2-tailed) The methods used in this study were the same as those used for similar tests in the

international literature reviewed in Chapter Two and the sample size was larger than most of

the studies presented

Chapter Four will outline the study results with the comparative and descriptive statistical

analysis

91

CHAPTER FOUR RESULTS

41 Introduction

This chapter presents the study findings and the analysis of results obtained The data for the

prevalence of visual acuity refractive errors near point of convergence accommodation

functions and heterophoria are presented in histogram while the descriptive statistics are

presented in tables

42 Prevalence of Vision Defects

The prevalence is an indication of the subjects that have either achieved or did not achieve the

pass fail criteria and is not a normal distribution of a particular condition The prevalence rate

of the vision functions is expressed in percentages The other data is expressed as mean (M)

and standard deviation (SD) and calculated for descriptive purposes at 95 confidence

interval (CI)

If an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants would be expected

421 Visual Acuity (VA)

i Dyslexic Group In total 32 of subjects in the dyslexic group had visual acuity

less than 69 while 68 had visual acuity of 66 and 65 The distribution of those

subjects with VA le 69 is as follows 65 (2 subjects) had VAs of 69 65 (2

subjects) had VA of 612 65 (2 subjects) had VA of 615 3 (1 subject) had

VA of 630 3 (1 subject) each had VA of 638 648 660 respectively (Fig

41)

ii Control Group In total 32 of subjects in the control group had visual acuity

92

less than 69 while 68 had visual acuity of 66 The distribution is as follows 3

(1 subject) each had VA of 69 612 615 624 while 65 (2 subjects) had VA

of 630 638 and 648 respectively (Fig 41)

Fig 41 Prevalence of Visual Acuity

The mean visual acuity for the right eye for the dyslexic group was 017 031 and 000

024 for the right eye for the control group There was no statistically significant difference

between the two groups (p = 029) (Table 41) The mean visual acuity for the left eye for the

dyslexic group was 020 033 and 000 024 for the control group There was no

statistically significant difference between the groups (p = 023) (Table 41) LogMAR acuity

of 020 is equivalent to 69 in Snellenrsquos notation (Appendix E Visual acuity conversion table)

Table 41 Descriptive Statistics for Visual Acuity Variable Group N ( 95 CI) Minimum Maximum p

Mean (LogMAR)

SD

VAcuityRE

LE

DyslexicControl

DyslexicControl

3131

3131

017000

020000

031024

033024

000000

000000

010090

100090

029

023

422 Refractive Error

i Dyslexic Group Twenty three percent (23) of the dyslexic subjects had

refractive errors while seventy seven percent (77) were emmetropic (defined as

93

retinosocpic findings of less than 05 myopia less than +075 hyperopia or less than

075 astigmatism) (Fig 42) The distribution is as follows 65 (2 subjects) had

myopia 65 (2 subjects) had hyperopia (both latent) As cycloplegia was not

used in assessing refractive error an estimate of the diagnosis of latent hyperopia

was made based on a high difference between the retinoscopic findings and the

subjective refraction results In the event that the increased retinoscopic finding

(that is more plus) did not blur the subjectsrsquo vision latent hyperopia was assumed

About 10 (3 subjects) had hyperopic astigmatism 65 (2 subjects) had

amblyopia (defined as visual acuity in either eye of 2040 or a two-line difference

in acuity between the two eyes with no improvement with pinhole) 141 and 65 (2

subjects) had anisometropia (defined as a difference of ge 075 in sphere or cylinder

between the two eyes) (Fig 43)

ii Control Group Twenty two and a half percent (225) of the participants from

the control group had refractive errors classified as follows 65 (2 subjects)

had myopia 3 (1 subject) had hyperopia and 13 (4 subjects) had astigmatism

and 65 (2 subjects) had anisometropia No participant had amblyopia

Fig 42 Prevalence of Refractive Errors (total

01020304050607080

Myopia HyperopiaAstimatism (Hyperopic)Anisometropia Amblyopia Emmetropia

Prevalence

Dyslexic GroupControl Group

0

5

10

15

20

25

Prevalence

Refractive Errors (total)

Dyslexic GroupControl Group

94

Fig 43 Prevalence of the Types of Refractive Errors

Refraction data for two children from the dyslexic group who had cataracts could not be

obtained due to poor reflexes and were excluded from the analysis

The mean spherical equivalent (SE) refraction for the right eye for the dyslexic group was 086

098 and 070 103 for the control group was There was no statistically significant

differences between the two groups (p=066) The mean spherical equivalent refraction of the

left eye for the dyslexic group was 057 101 and 049 109 for the control group There

was no statistically significant difference between the two groups (p= 092) (Table 42)

Table 42 Descriptive Statistics for Refractive Error Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

RE (SE)

LE (SE)

DyslexicControl

DyslexicControl

2931

2931

086070

057049

098103

101109

-125-350

-150-350

500350

400350

066

092

423 Near Point of Convergence

i Dyslexic Group Thirty three percent (33 10 subjects) had NPC break points

(objective) of greater than or equal to 10cm while about sixty seven percent 67

(20 subjects) had NPCs of less than 10cm (Fig 44) One child complained of being

tired and did not participate in this procedure and was therefore excluded

ii Control Group Forty eight percent (48 15 subjects) had NPC break point

(objective) of greater than or equal to 10cm Fifty two percent (16 subjects) had

95

NPC less than 10cm (Fig 44) Data on NPC for one child from the dyslexic group

was lost

The mean NPC break was 890 503 for the dyslexic group and 1260 870 for the control

There was a statistically significant difference between the two groups (p = 0049) The mean

NPC recovery was 1400 588 for the dyslexics and 2200 820 for the control group

There was a statistically significant difference between the two groups (p = 006) (Table 43)

Table 43 Descriptive Statistics for NPC Break and Recovery Points Variable Group N (95 CI) Minimum Maximum P

Mean SD

NPC Break

NPC Recovery

DyslexicControl

DyslexicControl

3030

3030

8901260

14002200

503870

588820

500400

600800

26003400

28003800

049

006

424 Ocular Health

Six and a half percent (2 subjects) in the dyslexic group had cataracts (unclassified) on both

eyes No pathology was detected in the control group

425 Heterophoria

i Dyslexic Group No subject in the dyslexic group manifested with a phoria at

96

distance

At near 3 (1 subject) had an esophoria greater than or equal to 4 prism diopters

3 (1 subject) had an esophoria of 3 prism diopters 65 (2 subjects) had

exophorias of greater than or equal to 4 prism diopters 3 (1 subject) had an

exophoria greater than or equal to 8 prism dioptres and 65 (2 subjects) had

exophorias of 6 prism diopters (Fig 45)

ii Control Group At both distance and near no subject in the control group had

phoria of greater than 2 prism diopters

Fig 45 Prevalence of Phoria at Near (40cm) with Maddox Wing

Four children from each group could not complete the test as they left to attend lessons The

mean exophoria for the dyslexic group was 163 261 and 180 042 for the control group

There was no statistically significant difference between the two groups (p = 059) The

mean esophoria for the dyslexic group was 350 070 and 2 000 for the control group

There was no statistically significant difference between the two groups (p= 046) (Table 44)

No tropias were observed

Table 44 Descriptive Statistics for HeterophoriaVariable Group N ( 95 CI) Min Maxi P

97

Mean SD

At NearExophoria

Esophoria

DyslexicControl

DyslexicControl

2727

2727

163180

350200

261042

070000

000100

300200

1000200

400200

059

0469

426 Accommodation Functions

a Accommodation Posture As shown in Figure 46 the distribution of accommodation

lag is as follows

i Dyslexic Group

Thirty nine percent (11 subjects) had a lag of accommodation (defined as + 075 or

greater) three and a half percent (1 subject) had lead of accommodation (defined as

- 025 or greater) while fifty seven percent had normal accommodation posture

ii Control Group

Forty two percent (13 subjects) had lag of accommodation About eleven percent

(11) (3 subjects) had lead of accommodation while about thirty nine percent had

normal accommodation posture

Three children from the dyslexic group could not continue with this test as they had to

leave to attend class activities while three children from the control group sought

permission to be out of the testing room at that point These children were excluded

from the analysis for binocular accommodation lag

98

Fig 46 Prevalence of Accommodation Lag ( ge+075)

The mean accommodation lag for the right eye was 091 038 for the dyslexic group

and 092 057 for the right eye for the control group There was no statistically

significant differences between the two groups (p =083) The mean for the left eye for

the dyslexic group was 085 036 and 091 048 for the control group There was no

statistically significant difference between the two groups (p = 061) (Table 45)

Table 45 Descriptive Statistics for Accommodative PostureVariable Group N (95 CI) Minimum Maximum P

Mean SD

Accomm lag

RE

LE

DyslexicControl

Dyslexic Control

2828

2828

091092

085091

038057

036048

000-050

-050-050

200200

125200

083

061

b Accommodative Facility As shown in Figure 47 the distribution of accommodation

facility was as follows

i Dyslexic Group

Forty six percent (46 13 subjects) had normal accommodative facility (defined as

0

10

20

30

40

50

Prevalence

Bi no cu lar A ccom m od ation Lag

D y s lex ic G r o u p

C on t ro l G ro u p

99

greater than 7cpm) binocularly at near while (54) (15 subjects) had

accommodation facility less than or equal to 7cpm (Fig 47)

ii Control Group

Sixty seven percent (67) (18 subjects) had normal accommodative facility

(defined as greater than 7cpm) while thirty three percent (33) (9 subjects) had

accommodation facility less than 7cpm

Three children from the dyslexic group and four from the control did not complete the test

as they indicated that they were tired The mean binocular accommodation facility was

686 cpm 274 for the dyslexic group and 885 369 for the control group There was a

statistically significant difference between the two groups (p=0027) (Table 46)

Table 46 Descriptive Statistics for Binocular Accommodative FacilityVariable Group N ( 95 CI ) Minimum Maximum p

Mean SDAccommodativeFacility at near

DyslexicBE

ControlBE

28

27

686

885

274

369

2

2

12

21

002

7

c Amplitude of Accommodation Monocular amplitude of accommodation in both eyes

and for both groups did not differ significantly (not more than 050D) except for one

subject who had a difference of 4 diopters between the two eyes The amplitude of

100

accommodation for two subjects who had latent hyperopia in the dyslexic group was

relatively low (600DS and 800DS) as compared to age minimum amplitude of

accommodation of 1175DS The amplitude of accommodation for two participants who

had cataracts were excluded from the analysis

Only the data for the monocular amplitude was analyzed When accommodation

amplitude is assessed monocularly it measures the response for each eye individually

and is particularly important to determine whether a patient has accommodative

insufficiency 133 The monocular amplitude of accommodation has been used to assess

amplitude of accommodation function in several studies 86 88100102-103 The mean for the

right eye for the dyslexic group was 1198 234 and 1287 108 for the control group

There was no statistically significant difference between the two groups (p =007) (Table

47) The mean for the left eye for the dyslexic group was 1214 214 and 1287 116

for the control group There was no statistically significant difference between the two

groups (p =022) (Table 47)

Table 47 Descriptive Statistics for Amplitude of Accommodation Variable Eye Group N ( 95 CI ) Minimum Maximum p

Mean SDAmplit of Acomm

RE

LE

DyslexicControl

DyslexicControl

2931

2931

11981287

12141287

234108

215116

810

810

2015

2015

007

022

d Relative Accommodation (NRA PRA) As shown in Fig 48 the results for the relative

accommodation were as follows

For the PRA all subjects in both groups had PRAs of more than -200DS For the

negative relative accommodation (NRA) 89 of the dyslexic subjects had NRA greater

than +200DS while 96 of subjects from the control group had NRA greater than

+200DS

101

Fig 48 Prevalence of Relative Accommodation (NRA+2PRA-200)

Three children from the dyslexic group and five from the control group could not

continue with the test for the assessment of relative accommodation as they had to attend

important class activities

The mean PRA for the dyslexic group was -623 117 and -606 063 for the control

group There was no statistically significant difference between the two groups (p

=051) The mean NRA for the dyslexic group was 322 079 and 311 047 for the

control group There was no statistically significant difference between the two groups

(p =068)(Table 48) The findings for RA were higher than the published norm This

may be due to the process of addition of trial lenses

Table 48 Descriptive Statistics for Relative Accommodation Variable Group N ( 95 CI ) Minimum Maximum p

Mean SDPRA

N RA

DyslexicControl

DyslexicControl

2826

2826

-623-606

322311

117063

079047

-900-700

200200

-400-500

60045

051

068

427 Fusional Reserves

The children in both groups either could not report or understand blur so the result for break

102

and recovery was used in all analysis of vergence function

a Base-in Vergence Reserves at Distance two children from the dyslexic group could not

complete all aspects of the fusional reserves assessment because they were tired These

children were excluded in the analysis

The mean base-in to break for the dyslexic group was 1469 683 and 1600 350 for

the control group There was no statistically significant difference between the two

groups (p =046) The mean base in to recovery for the dyslexics was 1172 620 and

1280 317 for the control group There was no statistically significant difference

between the two groups (p =049) (Table 49)

Table 49 Descriptive Statistics for Base-in Vergence Reserves at distanceVariable Group N (95 CI) Minimum Maximum P

Mean SD

BI to Break (dist)

BI to Recovery (distance)

DyslexicControl

DyslexicsControl

2931

2931

14691600

11721280

683350

620317

410

28

4022

3520

046

049

b Base-in Vergence Reserve at Near The mean base in to break at near was 1185 514

for the dyslexics and 1283 313 for the control group There was no statistically

significant difference between the two groups (p=029) The mean base in to recovery was

872 478 for the dyslexics and 1032 335 for the control group There were no

statistically significant differences between the two groups (p=017) (Table 410)

Table 410 Descriptive Statistics for Base-in Vergence Reserves for NearVariable Group N (95 CI) Minimum Maximum P

Mean SDBI to Break (near)

BI to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

11851283

8721032

514313

478335

26

14

2518

2015

029

017

103

c Base-out (BO) Vergence Reserves at Distance Four children from the control

group could not give a report on the recovery point Two children from the dyslexic group

could not complete the test The mean base out to break at distance for the dyslexic group

was 2706 925 and 2416 975 for the control group There was no statistically

significant difference between the two groups (p=024) The mean BO to recovery for the

dyslexic group was 1876 796 and 17 693 for the control group There was no

statistically significant difference between the two groups (p= 040) (Table 411)

Table 411 Descriptive Statistics for Base-out Vergence Reserves at Distance Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

BO to Break(Distance)

BO to Recovery(dist)

DyslexicControl

DyslexicsControl

2931

2927

27062416

18761700

925975

796693

10001000

400600

0004000

35003500

024

040

d Base-out Vergence Reserves at Near The mean base-out to break at near was 2160

1162 for the dyslexic group and 2109 842 for the control group There was no

statistically significant difference between the two groups (p =084) The mean base-out

to recovery at near for the dyslexic group was 1335 745 and 1555 625 for the

control group There was no statistically significant difference between the two groups

(p =016) (Table 412) The comparative and descriptive statistsics for all variables is

shown in Table 413

Table 412 Descriptive statistics for base-out vergences at near Variable Group N (95 CI) Minimum Maximum P

Mean SD

BO to Break (near)

BO to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

21602109

13351555

1162842

745625

8001000

600800

4040

3530

084

016

104

Table 413 Comparative Descriptive Statistics for Dyslexic and Control groups

Variable Dyslexics( 95 CI)

Control( 95 CI)

N Mean SD N Mean SD PVisual Acuity

RE 31 017 031 31 000 024 029LE 31 020 033 31 000 024 023

Refractive ErrorRE 29 086 098 31 070 103 066LE 29 057 101 31 049 109 009

NPC Break 30 890 503 31 1260 870 049NPC Recovery 30 14 588 31 2200 820 006

Heterophoria (Near)Exo 27 163 261 27 180 042 059Eso 27 350 070 27 200 000 046

PRA 28 -623 117 26 -606 063 051NRA 28 322 079 26 311 047 068

Amp of AccommodationRE 29 1198 234 31 1287 108 007LE 29 1214 215 31 1287 116 022

Accom Facility (bin) 28 686 274 27 885 369 003

Accomm LagRE 28 091 038 31 092 057 083LE 28 085 036 31 091 048 061

BI Break (Dist) 29 1469 683 31 1600 350 046BI Recovery (Dist) 29 1172 620 31 1280 317 049

BI Break (Near) 29 1185 514 31 1283 313 029BI Recovery (Near) 29 872 478 31 1032 335 017

BO Break (Distance) 29 2706 925 31 2416 975 024BO Recovery (Distance) 29 1876 796 31 1700 693 040

BO Break (Near) 29 2160 1162 31 2109 842 084BO Recovery (Near) 29 1335 7 45 31 1555 625 016

105

Summary Statistical tests were performed on the data to ensure scientific validity and were presented for

comparative and descriptive purposes The prevalence of visual acuity was similar between the

dyslexic and control groups and there was no statistically significant difference between the two

groups (p = 029 right eye 023 left eye)

The prevalence of total refractive errors was similar between the two groups and there was no

statistically significant difference between the two groups (p = 066 right eye and 092 left

eye) Hyperopia was slightly more prevalent in the dyslexic group than in the control group

The prevalence of myopia was the same in both groups Astigmatism was more prevalent in

the control group than in the dyslexic group The prevalence of anisometropia was the same in

both groups Amblyopia was more prevalent in the dyslexic group compared to the control

group

The control group had a higher near point of convergence break and recovery points than the

dyslexic group The NPC showed a statistically significant difference between the two

groups NPC break (p =0049) and NPC recovery (p =0006) The prevalence of exophoria at

near was higher in the dyslexic than the control group There was no statistically significant

difference between the two groups (p= 059) The prevalence of esophoria in the dyslexic

group was higher than that found in the control group but there was no statistically significant

difference (p =046)

The prevalence of relative accommodation was similar in both groups and there was no

statistically significant difference between the groups The dyslexic group had a reduced

amplitude of accommodation compared to the control group and there was no statistically

significant difference between the two groups (p=007 right eye andp = 022 left eye) The

dyslexic subjects performed significantly worse than the control subjects in accommodative

facility function and there was a significant difference between the two groups (p =0027)

The dyslexic subjects had a lower accommodation lag than the subjects from the control

group There were no statistically significant differences between the two groups right eye (p

= 083) and left eye (p = 060)

At near the negative fusional vergence (base in vergences)) was reduced in the dyslexics

compared to the control group while the positive fusional vergence (base out vergences) was

similar for both groups There was no statistically significant difference between the groups

106

(p = 021 for BI break at near) and (p = 084 for BO break at near) Having analyzed the

results Chapter Five will discuss the findings in the context of the international studies

reviewed in the literature

107

CHAPTER FIVE DISCUSSION

51 Introduction

A review of the literature reveals that studies 2356878990 conducted on dyslexic children have

investigated the prevalence of vision defects in the dyslexic population and examined whether

such vision anomalies are correlated with dyslexia by comparing vision characteristics in

dyslexic subjects to normal readers

The aim of this study was to determine the prevalence of vision defects in a South African

population of the dyslexic school children and investigate the relationship between vision and

dyslexia by comparing the vision characteristics of the dyslexic to the control group of learners

from a mainstream school The proposed null hypothesis was that there was no statistically

significant difference between the means of the vision functions in the dyslexic population

compared to the control group A discussion of the results of this study is presented

52 Prevalence of Vision Defects

521 Visual Acuity

The 32 prevalence of visual acuity worse than 69 was the same for both groups A reduced

visual acuity could be due to ocular abnormality (such as cataract) or due to normal refractive

error differences in the population Visual acuity defects due to ocular diseases are usually not

common in paediatric populations142 but tend to be related to refractive error changes in the

population143 This relates more in the present study as the prevalence of defects of visual acuity

and the total refractive errors were similar Consequently in relating visual acuity to refractive

errors myopia astigmatism and hyperopia may reduce distance or near visual acuity

108

Visual acuity testing is widely performed in screening refraction and monitoring of disease

progression It is also used in both basic and clinical vision research as a way to characterize

participants visual resolving capacities144 Specific to the present study visual acuity testing

helps detect visual problems at the distances at which most school learning activities occur and

gives useful preliminary information on how good or poor a childrsquos vision is reduced distance or

near visual acuity gives an indication of poor vision that will necessitate further examination

However in assessing how visual acuity relates to reading performance it is important to note

that visual acuity as measured with the traditional letter identification only assesses the ability to

discern letters on the visual acuity charts and does not assess dynamic vision behaviour

binocular eye control or ocular stress and so may not necessarily mean normal vision

Furthermore normal visual acuity may not necessarily mean normal vision since some people

may have other vision defects such as color vision or reduced contrast or inability to track fast-

moving objects and still have normal visual acuity The reason visual acuity is very widely used

is that it is a test that corresponds very well with the normal daily activities a person can handle

and evaluates their impairment to do them 145

In relation to previous studies the visual acuity findings in this study are similar to reports by

Buzzelli 6 Goulandris et al 90 and Metsing and Ferreira 103 On the contrary Hoffman 83 found

a lower prevalence (1028) of visual acuity worse than 69 in a group of learning disabled

children In contrast to the present study however the mean age of subjects in Hoffmanrsquos study

was 8 years 6 months while the mean age in the present study was 13 for dyslexics and 11years 9

months for the control group Grnlund et al 140 documented that age must be taken into

consideration when describing and comparing VA in different populationsbecause VA develops

from birth to adolescence A lower prevalence (10) of visual acuity worse than 69 was also

reported by Sherman84 Latvala et al 3 reported a slightly worse distance acuity in the dyslexic

group than in the control group but noted no statistically significant difference Evans et al 5

found that dyslexic groups had a significantly worse visual acuity than the control group

(Binocular distance VA p =00164 and binocular near VA p =00018) Similarly Ygge et al2

reported that the subjects from the control group had a better visual acuity than the dyslexic

group at both distance and near and that the results showed statistically significant differences

(distance at p =003) (near at p=0005) Bucci et al 89 and Kapoula et al 87 reported the

109

prevalence of visual acuity of all participants in their study to be normal A study conducted by

Grisham et al 86 lacked a control group but it was reported that 568 of the participants had 66

acuity in both eyes (compared to 68 in dyslexic group in the present study) 172 had 69 or

worse (compared to a total of 315 who had visual acuity of 69 and worse in the present study)

The mean age of the participants in the study by Grisham et al 86 was 154 years while the mean

age of the participants in the dyslexic group in the present study was 13 years

The factors which may affect the outcome of visual acuity measurements that may lead to

variations in results reported by different authors include size position and distribution of retinal

mosaics (retinal photoreceptors) 143 optical aberration pupil size clarity of the optical media

age method and type of chart used contrast isolated or multiple letters state of accommodation

illumination as well as the criteria used to define visual acuity114 140 Psychological factors

affecting visual acuity testing results include blur interpretation fatigue and malingering 143

These factors may be difficult to control and could constitute confounding variables which may

lead to variations in visual acuity results reported by different authors

522 Refractive Error

The prevalence of uncorrected refractive error of 23 found in the dyslexic group was similar to

the 225 found in the control group and there was no statistically significant difference between

both groups (p = 066 right eye p = 092 left eye) This finding may mean that the dyslexic

subjects are not at more risk of a particular refractive anomaly compared to participants from the

control group as the dyslelxic gourp has similar distribution of refractive errors

Some studies 5838498103146 did not classify refractive error according to the types but presented

the results for the total refractive error On this basis the prevalence of total refractive error

(23 for dyslexic and 225 for the control group) in the present study is similar to reports by

other studies Hoffman83 reported a 215 prevalence of refractive error in a study of the

incidence of vision difficulties in learning disabled children Although there was a marked

difference in the sample size in both studies the similarities in the findings may be because as

with the present study Hoffmans study was based on a population of learners in a special school

110

referred for optometric care by educators psychologists and reading specialists

Similar to the present study Evans et al 5 and Ygge et al 2 reported that the prevalence of

refractive errors in the dyslexic and control groups was similar Metsing and Ferreira103 found

no statistically significant difference between the learning disabled and the mainstream

groups in their study Contrasting findings were reported by Sucher and Stewart 98 who found

the prevalence of refractive errors to be 94 for the control group although the 208 found in

their dyslexic group is similar to the 23 found in the present study Wesson 146 reported the

prevalence of refractive error in children with reading dysfunction to be 38 while the

prevalence of refractive errors was 16 in the study by Sherman 84 In the study by Alvarez and

Puell 88 the prevalence of refractive error was 424 in the dyslexic group while 415 of the

control group had refractive error When compared to the present study the higher prevalence of

refractive error in the dyslexic group can be accounted for by the relatively high prevalence of

hyperopia (287) and in the control group the higher prevalence of refractive error can be

accounted for by the high prevalence of myopia (194) It is unclear why the prevalence of

hyperopia was as high as no information was given on how refractive error was assessed

Statistically for the present study the mean refractive errors for the dyslexic group were right

eye 086 098 left eye 057 1013 and right eye 070 103 left eye 049 109 for the

control group There was no statistically significant difference (RE p =066 LE p = 092)

Evans et al 91 reported a mean spherical refractive error of 077DS (right and left eye similar) In

the study by Alvarez and Puell 88 the refractive error results (descriptive statistics) reported

were as follows for the poor readers mean spherical equivalent refractive errors for the right and

left eye were 020 06 and 020 06 respectively In the control group mean spherical

equivalent refractive errors for the right and left eye were - 020 08 and - 014 08

respectively The descriptive statistics on refractive errors was not reported in the studies 3 93

cited earlier which made it impossible to compare the statistical findings with the present study

In the present study about 68 of participants from both groups were emmetropic (defined as lt -

050DS +100DS and 075D cyl) while 655 of the dyslexic children and 645 of children

from the control group were emmetropic in the study by Alvarez and Puell 88 Only the study by

111

Alvarez and Puell 88 reported on emmetropia

Refractive errors are among the leading causes of visual impairment worldwide and are

responsible for high rates of visual impairment and blindness in certain areas School children

are considered a high risk group because uncorrected refractive errors can affect their learning

abilities as well as their physical and mental development147

Several factors may lead to variations in results found in different studies This include the type

of population studied (clinical or non-clinical) sampling method (convenience or cluster)

classification criteria examiner bias and more specifically the use of cycloplegia in the

assessment of refractive errors

The following studies2 3 8895 categorized refractive errors as hyperopia myopia astigmatism

anisometropia or amblyopia

a Hyperopia In the present study hyperopia appeared slightly more prevalent (65) among

the dyslexic group than in the control group (3) This result agrees with the report from

other studies 93 95 and Alvarez and Puell88 who reported a much higher prevalence (287)

for the dyslexic group compared to 161 of the control group Helveston85 found no

difference in the prevalence of hyperopia between the population of normal and poor

readers

Hyperopia is the refractive error that is consistently reported 3939598 to be associated with

reading difficulties so it was expected that one will find the higher prevalence of hyperopia

in the present study

In the present study two children had latent hyperopia (based on the assumption that

increased plus did not blur the distance vision) The full magnitude of the hyperopic

findings could not be estimated because cycloplegia was not used Therefore it is possible

that the prevalence of hyperopia may have been under-estimated Williams et al 148 studied

the ldquoprevalence and risk factors for common vision problems in childrenrdquo and concluded

112

that the 48 prevalence of hyperopia in their study was ldquoan underestimate as data was

collected without cycloplegiardquo Borghi and Rouse (cited by Scharre and Cree)149

determined that on an average 063D more plus was identified by cycloplegic refraction

In relating hyperopia to reading difficulties simple to moderate hyperopia may not cause

constant blur at a distance or near point but the extra accommodative effort produces

asthenopic symptoms of intermittent blur headache fatigue and loss of concentration and

inattention in some patients which may be mistaken for short attention span Uncorrected

hyperopia is associated with esophoria at near point which can stress the fusional vergence

systems that hold the eyes in correct alignment If the hyperopia and esophoria is excessive

an accommodative esotropia can result 81150 This may result in difficulty in reading

b Myopia The 65 prevalence of myopia was the same in both groups This result is similar

to an equal prevalance of myopia reported by Eames 95 (4 in both groups) Alvarez and

Puell 88 reported that 57 of dyslexics were myopic while 194 of the control group was

myopic The prevalence of myopia was higher in studies by Rosner and Rosner 93 (54

non- learning disabled and 19 learning disabled) and lower than the prevalence reported

by Grisham and Simons150 In contrast to the present study subjects from the study by

Rosner and Rosner 93 were patients of an eye clinic where the two main reasons for referral

were reduced visual acuity and school learning problems while subjects for the present

study were referred to the special school mainly due to low academic performance which

may not necessarily be vision- related The subjects were chosen for the study irrespective

of whether they complained of a visual impairment or not whereas in the study by Rosner

and Rosner 93 study subjects had been identified as having a visual impairment Therefore

the higher prevalence may have been from the selection protocol used by Rosner and

Rosner 93 in their study

The prevalence of myopia was not reported in several studies 3568498146 referenced but was

presented as ldquorefractive errorsrdquo (was not classified as myopia hyperopia or astigmatism)

The onset and progression of myopia may be influenced by factors such as environment

nutrition genetic predisposition premature and low birth weight the effect of close work

113

racial and cultural factors differences in pupil size and illumination143151 Variations in the

prevalence of myopia reported by different authors may be related to the variation in the

above-mentioned factors

c Astigmatism

The prevalence (13) of astigmatism in the control group was higher than in the dyslexic

group (10) and there was no statistically significant difference between the two groups

Similarly Alvarez and Puell 88 found a higher prevalence of astigmatism in the control

group compared to the dyslexic group astigmatism was detected in 161 from the control

group compared to 8 and 92 right and left eyes respectively from the dyslexic group

In contrast Latvala et al 3 reported that 36 in the dyslexic group had astigmatism and

none from the control group had astigmatism Ygge et al 2 reported that the prevalence of

astigmatism was higher in the dyslexic group (28 left eye 25 ) compared to the control

groups (right eye 183 left eye 243) although there was no statistically significant

difference between the groups (p=025)

Rosner and Rosner93 reported that 30 of non-learning disabled and 27 of learning

disabled participants had astigmatism which differed markedly from the findings in the

present study despite the similarities in the study population Eames95 found an equal

prevalence of astigmatism (7) between the two groups studied

Astigmatism affects vision in different ways Clinically astigmatism over 150 diopters can

often cause severe eye strain and interfere with reading and even lesser degrees of

astigmatism can be symptomatic in some patients 81

Naidoo et al 157 studied refractive error and visual impairment in African school children in

South Africa Although the study was conducted specifically on mainstream school

children it relates to the present study as it provided useful information on the visual

characteristics of school children in South Africa and forms a basis for comparison since all

the studies reviewed in the present study were conducted on Caucasian population except

114

for the study by Metsing and Ferreira 103 which did not provide much information on

refractive errors in the their study of learning disabled children in Johannesburg South

Africa

Naidoo et al 157 reported a prevalence of hyperopia of 18 myopia of 29 astigmatism

of 67 and 68 right eye and left eye respectively In comparison the prevalence of

refractive error for the control group in the present study was hyperopia 3 myopia 65

and astigmatism 13 The refractive error trend (astigmatism being higher in prevalence

than myopia and hyperopia) was similar between the present study and the study by Naidoo

et al 157 despite the difference in prevalence reported Given that ethnic origins culture

socio-economic class are comparable between both studies the difference in prevalence

may (partly) have been due to different sampling methods (cluster versus convenience) and

the use of cycloplegia in their study The use of cycloplegia has been reported to yield

spherical aberrations and unpredictable errors due to associated mydriasis 110

It has also been documented 158 that the use of cycloplegia yields more positive retinoscopic

findings although this did not seem to apply in this comparison as the prevalence of

hyperopia in the present study was higher than that reported by Naidoo et al 157 A possible

explanation for the difference in prevalence may be related to the different criteria used to

define hyperopia Bennet et al 159 stated that the degree of ametropia ldquohellip considered to be

within normal limits in a particular study will drastically affect the prevalencerdquo Similar

opinion was noted by Gronlund et al 140 who stated that ldquothe choice of definition will of

course have a great impact on prevalence In their study on Swedish children aged 4-15

years Gronlund et al 140 defined astigmatism as ge 075 and stated that the prevalence of

astigmatism would have been 21 (instead of 32) had they defined astigmatism as ge

100 Furthermore Grisham et al 86 in their ldquoStudy of poor readers in high schoolrdquo noted

that helliprdquoif one selected less stringent criteria then the proportion of students with any given

dysfunction would decreaserdquo In the present study the criteria used to classify hyperopia

were ge +100D while Naidoo et al 157 used ge +200DS cut- off

115

d Anisometropia Anisometropia may be defined as a difference of 075 (sphere or cyl) or

more between the two eyes and this definition was the criteria used in this study 129

Anisometropia is of great clinical interest because of its intimate association with

strabismus and amblyopia141 152

The 65 prevalence of anisometropia was similar in both groups This is comparable to

results reported by Latvala et al 3 where the prevalence of anisometropia was 36 (2

subjects) for the dyslexic group and 6 (3 subjects) for the control group but contrasts with

findings by Eames 95 who reported the prevalence of anisometropia to be 13 for poor

readers and 6 for the control group while Ygge et al 2 found a higher prevalence of

anisometropia in the control group (158) than in the dyslexic group (94)

In anisometropia the difference in refraction as well as the refractive error causes the image

to be out of focus on one retina blunting the development of the visual pathway in the

affected eye 141 The fovea of an anisometropic eye receives images from the same visual

object however the images from the more myopic or hyperopic eye are out of focus 153 It

appears that anisometropia is a major cause of amblyopia for at least one third of all

amblyopias 131 and anisometropia is considered to be a causal factor in the pathogenesis of

amblyopia and strabismus It is also estimated that 6 to 38 of all cases of amblyopia are

caused by anisometropia without strabismus whereas in about 12 to 18 of the children

with strabismus this is accompanied by anisometropia 154

According to Grosvenor 114 uncorrected anisometropia even of a moderate amount may

induce eyestrain because it is impossible for the accommodation mechanism to maintain

clear images on the retina at the same time On the other hand large amounts of

anisometropia (2DS or more) may not cause symptoms as no effort is made to maintain

single binocular vision 114

Rutstein and Corliss155 investigated the relationship between anisometropia amblyopia and

binocularity and concluded that higher degrees of anisometropia generally cause deeper

116

amblyopia and poorer levels of binocularity for hyperopes but not for myopes and stated

that ldquoAlthough uncommon small amounts of anisometropia can cause moderate

amblyopiardquo However the results of the present study failed to support this claims as none

of the subjects had amblyopia resulting from anisometropia

In anisometropia the displacement or distortion of the image prevents the development of

fine visual perception in the occipital cortex and puts the child at risk for developing

amblyopia 141 Anisometropia causes poor reading skills probably through the mechanism

of poor sensory and motor fusion rather than reduced visual acuity It degrades binocular

coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 150

e Amblyopia Two subjects (65) in the dyslexic group had ambyopia due to cataracts and

none from the control group had amblyopia This result may be comparable to studies by

Latvala et al 3 who reported 36 (2 subjects) for the dyslexic group and that of Rosner and

Rosner 93 with a prevalence of 4 for the non-learning disabled group and 3 for the

learning disabled group Cataract is a major cause of amblyopia (deprivation ambyopia) by

causing an impediment to the visual axis 141 Other conditions that may lead to amblyopia

include strabismus (which causes the images on the retinas to be dissimilar) and uncorrected

refractive errors (especially as in anisometropia) Anisometropia is related to amblyopia

Amblyopia is a major public health problem It is the most common cause of monocular

vision loss in children and young adults Early recognition and prompt referral are crucial

especially during infancy and childhood to prevent permanent loss of

vision 141

523 Heterophoria

Heterophoria is the relative deviations of the visual axes when the eyes are dissociated 160 Some

degrees of heterophoria are considered normal for persons with normal binocular vision

Approximately 1-2 prism dioptres of esophoria or 1- 4 diopters of exophoria at distance should

117

be considered to be within normal limits161 At near 3-6 prism dioptres of exophoria is

considered normal (physiological exophoria) 161 A normal healthy eye is usually able to

overcome these small deviations and so it is described as being compensated161 However

esophoria is much less compatible with comfortable vision than is exophoria Any amount of

esophoria in a patient with visually related symptoms may be problematic 162

In the present study all subjects were orthophoric at distance This report corroborates reports by

other authors Mathebula et al 163 studied heterophoria in a South African population of school

children (mean age 10 16 years) and reported that the mean distance horizontal heterophoria

showed orthophoria Similarly Evans et al 100 reported that all subjects examined ldquowere

orthophoric at distance with cover testrdquo According to Dowley160 ldquothe high prevalence of

orthophoria is realrdquo and that ldquothe high prevalence of distance orthophoriahellip may be due to the

coordinating influences of the same mechanisms that subserves prism adaptationrdquo A similar

view was expressed by Walline et al 164 who studied the ldquodevelopment of phoria in childrenrdquo and

stated that ldquonearly all of our subjects were orthophoric at distancerdquo

The major findings in heterophoria was a 95 prevalence of exophoria at near which was more

in the dyslexic subjects than in the control group with no exophoria There was no statistically

significant difference between the two groups (p= 059) At near 3 (1 subject) of the dyslexic

group had an esophoria greater than or equal to four prism diopters An interpretation of the

above finding is that the higher prevalance of heterophoria may mean that the dyslexic subjects

may be more uncomfortable when doing near work than the nroaml readers A possible

explanation may be that according to von Noorden (cited by Kommerell et al)165 heterophoria

typically causes asthenopia Patients with asthenopic symptoms (feeling of heaviness dryness

and soreness of the eyes pain in and around the eye frontal and occipital headache and the eyes

are easily fatigued) often have an aversion to reading and studying Typically such complaints

tend to be less severe or to disappear when patients do not use their eyes in close work165 It has

been documented that decompensated heterphoria (heterophoria accompanied by symptoms) can

be due to poor fusional reserves problems with sensory fusion (such as in anisometropia) or may

be due to an unusually large phoria which may be due to anatomical reasons or uncorrected

hyperopia 161166 The presence of phoria could contribute to visual and attention abnormalities

118

noted among children with reading difficulties making near point visual activities more

strenuous

Nearpoint esophoria has been reported to be associated with childhood myopia progression 164167

In the present study the only subject that had four prism diopters esophoria at near in the

dyslexic group had +125 hyperopia while the subject that had 3 prism diopters esophoria had -

150 DS myopia However due to the small sample size in the present study it is difficult to

draw any conclusion on the association between myopia and esophoria

The findings on heterophoria as in other aspects of vision functions in the dyslexic populations

reviewed are mixed However similar to the present study Latvala et al 3 assessed near

heterophoria using the Maddox Wing and reported a higher prevalence of near exophoria in the

dyslexics than in the control group Similarly Kiely et al 12 documented that dyslexics tend to

be more exophoric than normal readers Evans et al 101 found a higher prevalence of lateral

phoria in learning disabled subjects compared to normal readers In contrast to the above

findings Sucher and Stewart 98 assessed phoria using the cover test (testing distance was not

indicated in the results) but found a slightly higher prevalence of phoria in the control group than

in the dyslexics Evans et al 100 assessed near horizontal phoria using the Maddox Wing as in

present study but found no difference in near horizontal phoria between the dyslexic and control

groups Bucci et al 89 reported that there was no difference in phoria results at both distance and

near between the dyslexic and control groups Metsing and Ferreira 103 found no statistically

significant difference between the learning disabled and the mainstream groups on

heterophoriaThe use of the cover tests for both distance and near to assess heterophoria may have allowed for

standardization in technique but may not have implied that more valid results will be obtained

The cover test and the Maddox Wing have been used simultaneously to assess heterophoria in

other studies3 100 The possible sources of variations in results reported by different investigators

include classification criteria and poor technique 168

524 Near Point of Convergence

119

In the present study the prevalence of a remote (greater than 10cm) near point of convergence

was higher in the control group with 48 than in the dyslexic group with a prevalence of 33

There was a statistically significant difference in the near point of convergence break (p=0049)

and recovery (p = 0006) between the two groups

It may be that children who do not have reading difficulties tend to read more often than children

who experience difficulty to read This is because with increasing ability to read there is likely

to be more demand on accommodation and convergence resulting in near point stress as well as

esophoria The normal readers use and exercise the accommodation and convergence system

more frequently According to Owens and Wolf-Kelly 169 near work has long been considered a

potential source of visual problems stating that in their study reading ordinary text at a near

distance for about one hour induced significant changes in the resting postures of both

accommodation and vergences The authors indicated that ldquonear work induces a recession of the

near point of accommodation or vergencesrdquo

The near point of convergence break is associated with changes in inter-pupillary distance with

age As inter-pupillary distance widens with physical growth the amount of convergence in

prism diopters increases for a given distance from the subject 132 Chen et al 170 studied near point

of convergence in children aged 1-17 years and reported that an increasing incidence of remote

near point of convergence with increasing age in their study might be due to the near work

demands of primary school which might create a different level of near point stress than the near

work conditions in pre- primary school years

Similar to the findings in the present study Evans et al 91 reported a statistically significant

relationship between NPC and reading retardation (p=0019) Contrasting findings were reported

by other authors Bedwell et al 171 studied visual and ocular anomalies in relation to reading

difficulties and reported that 466 of the group of ldquogood readersrdquo had abnormal near point of

convergence compared to 58 of ldquopoor readersrdquo Similarly Bucci et al 89 and Kapoula et al 87

reported that the near point of convergence was significantly more remote in dyslexics than the

control In the study by Latvala et al 3 1275 of the dyslexic group and only 2 from the

120

control group had a near point of convergence of worse than 8cm A statistically significant

difference between the two groups was reported (p=00385) Metsing and Ferreira 103 found

no statistically significant difference between the learning disabled and the mainstream

groups on the near point of convergence function In a study that lacked control subjects

Grisham et al 86 reported a lower prevalence (154) of NPC (9cm or further) in a group of high

school poor readers The authors measured NPC three times on each subject in other to detect

fatigue but stated that it should therefore be noted that fatigue may be a problem for poor

readers even if they are able to converge at 8 cm if they tire easily then the ability to sustain

convergence during longer periods of reading may suffer

In the present study the measurement of the near point of convergence was repeated three times

on each subject in order to detect visual fatigue that the children experience everyday This

approach was documented by other authors 868789 111-113 Some authors111-113 have recommended

the measurement of the near point of convergence several times in order to detect fatigue which

is often indicative of poor convergence fusion system which may affect distract a child from

reading 172

525 Accommodation Functions

a Accommodation Facility Accommodative facility assesses the rate at which

accommodation can be stimulated and inhibited repeatedly during a specific time period It

relates to the individualrsquos ability to shift focus quickly and efficiently for varying distances

and is extensively used in the reading process 6

Only the result for the binocular accommodative facility was recorded and was used for analysis

in the present study Siderov and Johnston 174 noted that monocular accommodation

measurements provides a direct evaluation of the dynamics of accommodative response while

binocular testing of accommodative facility provides similar information but also reflects the

interactive nature of the relationship between accommodation and vergences

For the dyslexic group 54 (15 subjects) had inefficient accommodation facility while only 33

121

(9 subjects) of the control group had inefficient accommodative facility Statistically the control

group had a better accommodative facility (885cpm 369) than the dyslexics (686cpm 274)

and there was a statistically significant difference between the two groups (p =0027) It has

been documented 100 that an abnormal accommodative facility could imply difficulty in changing

focus from far to near and subsequently lead to a lack of interest in learning An efficient facility

of accommodation is particularly important because a greater period in school involves changing

focus between the chalkboard to near task such as writing at a desk Low values of

accommodative facility have been associated with symptoms related to near point asthenopia134

The result of the present study corresponds with findings by Evans et al 100 who reported that

dyslexics appeared to be slower at a test of accommodative facility Similar results were reported

in other studies 128898 but contrasts with results reported by Buzzelli 6 Buzzelli 6 assessed

accommodation facility using a Bernell Vectogram that utilized a polarised target to control for

suppression Buzzelli6 assessed accommodation facility as follows patients were asked to clear

the number 9 on the Bernell Vectogram target at 40cm They alternately viewed the target

through a 2D lens The test was continued for 20 cycles (a cycle being clearing the plus and

minus lens each presented once) The total time in seconds was recorded for the right eye left

eye and both eyes The polarized target controlled for suppression The use of the vectogram

to assess accommodation facility by Buzzelli6 differs from the flipper technique that was used in

the present study Secondly there was no control for suppression in the present study The

difference in results found between the two studies may be due to the lack of control for

suppression in the pre sent study Siderov and Johnston174 stated that ldquoclinical measurements of

binocular accommodation facility could vary depending on whether or not suppression has been

monitoredrdquo

A total of 317 of the poor readers in the study by Grisham et al 86 had inadequate (9cm or less)

accommodative facility

Allison173 suggested that there are different norms submitted by different authors for

accommodative facility measurements and that apart from adhering strictly to recommended

norms for accommodative facility testing other factors to be considered when evaluating

accommodative facility include the difficulty and speed of responses to the plus and minus lenses

122

and whether the subjects fatigue easily 173 Kedzia et al (cited in Zadnik)175 documented that

the factors that affect the testing of accommodative facility include the size and position of the

text being read the complexity of the target reaction time of the child to call out the symbol and

the magnification or minification factors introduced by the lenses themselves The above

variables are factors to be considered in assessing the variations in the findings for

accommodative facility reported in this study as compared to other studies

b Amplitude of Accommodation Accommodation is the ability to adjust the focus of the

eye by changing the shape of the crystalline lens to attain maximum sharpness of the retinal

image of an object of regard The absolute magnitude of the accommodative response is

termed the accommodative amplitude 120

Statistically the dyslexic group had a slightly reduced monocular amplitude of

accommodation (mean 12D) compared to the control group (1287D) but there was no

statistically significant difference between the two groups (p=070) The slight difference

in the mean amplitude of accommodation between the two groups may have been due to the

two subjects from the dyslexic group who had latent hyperopia

Similar to the present study lvarez and Puell88 reported that monocular accommodative

amplitude was significantly lower in the group of poor readers Evans et a l91100 in two

seperate studies 91100 reported that amplitude of accommodation was reduced in the

populations studied Ygge et al 102 and Goulandris et al 90 found no statistically significant

difference between the dyslexic and control groups Grisham et al 86 found that 247 of

the children had amplitude of accommodation which the authors classified as ldquoweakrdquo or

ldquovery weakrdquo while 115 had accommodative amplitudes classified as ldquoborderlinerdquo In the

present study the mean amplitude of accommodation for each group was within normal age

norms according to the amplitude norms and Hofsteterrsquos formula 106161

In the study by Metsing and Ferreira 103 a higher percentage of low amplitude of

accommodation for the right and left eyes respectively (516 and 533) was found in the

mainstream group compared to 291 and 288 in the learning disabled group The

123

relationships between the mainstream group and reduced amplitudes of accommodation of

the right (p=004) and left eyes (p = 0001) were found to be statistically significant

(plt005) Furthermore the relationship between the mainstream group and reduced ampli-

tude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to be moderate and

that between the mainstream group and the right eyes (Cramerrsquos = 0286) to be low

When accommodation amplitude is assessed monocularly it measures the response for each

eye individually and the limiting factor is the magnitude of blur-driven accommodation

Monocular amplitude measures are particularly important to determine whether a patient

has accommodative insufficiency 107 The amplitude measured binocularly is usually

greater than the monocularly measured amplitude due to the influence of both the blur-

driven accommodative response and accommodative and vergence responses 120 175

Clinically the amplitude of accommodation is considered important mainly when it falls

below the expected age norm in which case the child may experience blur vision at near

Secondly a difference of up to 2D between the two eyes may also be considered clinically

significant 120

An inefficient accommodation function may lead to difficulties in learning as the focusing

system of the eyes play a major role in the learning process Children who suffer some

anomalies of accommodation are more prone to fatigue quickly and become inattentive than

those who have norrmal accommodation function 87 Symptoms of accommodation

insufficiency are specifically related to near vision work 176

Factors affecting the measurement of amplitude include differences when measurements

are taken monocularly as compared to when assessed binocularly the angle of gaze target

size age refractive error race and climate 175

c Accommodation Posture During near vision the eyes are not usually precisely focussed

on the object of regard but the accommodation lags a small amount behind the target If

the accommodation lag is small then the blur it causes is insignificant if high then it can

result in blurred print during reading 142

124

About 3928 (11 subjects) of dyslexic subjects had lag of accommodation compared to

4193 (13 subjects) of the control group The mean accommodation lag was similar in

both groups 092 for control and 088 for the dyslexic group There were no statistically

significant differences between the two groups right eye (p = 083) and left eye (p = 060)

Similarly Evans at al 100 reported that the mean accommodation lag did not show any

statistically significant difference between the dyslexic and control groups On the

contrary Metsing and Ferreira 103 found a high prevalence of lead of accommodation for

the mainstream group but a high prevalence of lag of accommodation in the learning

disabled group The relationships were found to be statistically significant (plt005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000) In another

study Evans et al 91 reported a mean accommodation lag of + 112DS

An individual with a lag of accommodation habitually under accommodates and may lead

to difficulty with reading An accommodative response that manifests as an excessive lag

of accommodation may indicate latent hyperopia esophoria or may be associated with

accommodative insufficiency or accommodative spasm poor negative vergences or when

patient is overminused 177 The prevalence of high lag may be related to the prevalence of

latent hyperopia and esophoria in the present study

C Relative Accommodation (PRA NRA) The relative accommodation tests assess the

patientsrsquo ability to increase and decrease accommodation under binocular conditions when

the total convergence demand is constant67 It is also an indirect assessment of the

vergence system since the vergence demand remains constant while accommodative

demand varies 121 The results for the relative accommodation for all subjects from both

dyslexic and control groups were unexpectedly high The norm for the mean NRA and PRA

were given as approximately +2 or - 2 diopters from two reports consulted 119 121 while the

relative accommodation results in this study ranged between 3 and - 6 diopters Similarly a

higher relative accommodative value was reported by Chen et aland Abidin139 in a study of

vergence problems in Malay school children The only available study accessed that

125

reported on relative accommodation was conducted by lvarezand Puell 88 who reported

that the negative and positive relative accommodation values were similar in both groups of

children Statistically the results were (NRA Control group 19 06 poor readers 19

06 PRA Control group 23 09 Poor readers 20 13) compared to the findings from

the present study (NRA Control group was 311 047 NRA dyslexic group 322 079)

(PRA Control ndash 606 063 PRA dyslexic was -623 117) The p values were not

indicated

The higher values found in the present study may be due to the process of addition of trial

lenses specifically The phoropter has been reported to be a better technique to assess

relative accommodation 119

Although the relative accommodation was not assessed by many authors different opinions

were expressed regarding the assessment of relative accommodation Latvala et al 3 noted

that positive and negative relative accommodation ranges are ldquodifficult to take and

unreliablerdquo According to Morgan 178 the PRA and NRA results are often unusually high

but recommended that such tests results can be used as a sort of indicative way and can only

be interpreted loosely and suggested that the use should be restricted to special

circumstances Garcia and Fransisco 122 reported that high NRA can be associated with

disorders such as accommodative insufficiency and convergence excess while high values

of PRA are related to anomalies in which accommodative excess appears but concluded that

PRA and NRA findings were used mainly as complementary diagnostics tests of some

disorders in literatures consulted

A high value of the negative relative accommodation could indicate that the children were

exerting maximum accommodation effort Because 250D of accommodation is exerted at

the 40cm working distance the maximum amount of accommodation that would be

expected to relax accommodation at 40cm would be 250D A value greater than 250D

would mean that accommodation may not have been fully relaxed during the subjective

refraction 114 Generally a high value of the NRA may also mean that the refraction may

126

have been under corrected for hyperopia or over corrected for myopia 122 Garciaand

Francisco122 reported the relationship between high NRA values and under corrected

hyperopia to be due to the etiology of hyperopia

According to Grosvenor 114 the amplitude of accommodation is a limiting factor for the

expected value of the PRA For example ldquoIf a patient has an amplitude of accommodation

of only 150D we would expect the minus-lens-to-blur test to be no greater than -150Dhelliprdquo

A high value of the positive relative accommodation is related to convergence insufficiency 122 Illumination and the range of fusional vergence also affect the relative accommodation

An excessive illumination will give an erroneously high finding due to the increased depth

of focus 100

Dysfunctions of accommodation can significantly interfere with the comfort clarity speed

and accuracy of reading as the child develops reading skills 51 Wick and Hall 120 suggested

that the results for the tests of accommodation are more meaningful when analyzed

together as the results of individual accommodation function may not give a true reflection

of the childrsquos accommodation dysfunction

526 Fusional Reserves

Fusional vergence amplitudes is a clinical measurement of a person`s fusional vergence ability It

reflects the ability of the oculomotor system to maintain sensory fusion in spite of varying

vergence requirements 179 The horizontal vergence reserves describe the amount by which the

eyes can be converged or diverged whilst the subject maintains fusion 100161 It is a measure of

how much fusional vergence is available in reserve that can be used to overcome a phoria The

amount of base out prism required to produce diplopia is called positive fusional reserves

(measures convergence) 100 The measurements of convergence fusion are also referred to as the

lsquoamplitude of fusional reserves 111 Clinically the fusional amplitude provides information about

a patientrsquos ability to maintain comfortable binocular vision 100112

The participants from both the dyslexic and control groups either could not report or understand

127

blur so the result for break and recovery was used in all analysis of vergence function Evans et

al 100 reported similar observations For the fusional reserves only the findings for the near

positive fusional vergence (convergence) are emphasized The positive fusional

reservesvergence (measures convergence ability) may be more important in assessing reading

dysfunction More so it has been suggested by other authors 112121180 that near measurements

are more relevant in assessing vision functions in dyslexic children

Based on Bishops 111 recommended norm of between 30 and 40 prism diopters for positive

fusional amplitude at near the prevalence of abnormal positive fusional vergence at near was

83 for the control and 74 for the dyslexics This appears unusually high Walters 168

explained that a possible reason for an unexpected high base out finding is that exophoric

conditions makes compensating for base out more difficult than base in testing It is unclear if

this is the case in the present study although the prevalence of exophoria at near in the present

study was 95 The compensation for a phoria is maintained by fusion reflex so knowing the

magnitude of the vergence amplitude that is needed to compensate a given phoria is very useful

clinically181

For the present study at near the negative fusional vergence (base in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base out

vergences) was similar for both groups Overall there was no statistically significant difference

between the groups (p = 021 for BI break at near) and (p = 084 for BO break at near) Evans et

al 100 reported that the dyslexics have reduced fusional reserves compared to the control group

Bucci et al 89 reported reduced divergence capabilities in dyslexics at near

When assessing fusional reserves the blur point is a function of the flexibility between vergence

and accommodation When little flexibility exists the addition of even low prism causes a

simultaneous accommodation shift with resultant blur Lack of low blur findings may indicate an

inadequate flexibility bond between accommodation and convergence The break point is an

indication of the quality of binocular function Break values will be adequate when fusion ability

is good while the recovery measure is a more subtle indicator of the quality of binocular function

Consequently reduced blur break or recovery findings indicate the presence of near point stress

128

182 Some subjects from both groups had break values higher than 40 prism diopters for base in

and base out reserves Wesson et al 125 in their study reported a similar observation on objective

testing of vergence ranges Such high break findings may be because suppression was not

controlled while assessing the fusional reserves in the present study When suppression is

controlled the average vergence values will be lower because the test is stopped when the

suppression is detected If suppression is not monitored the break is not detected until the

stimulus is outside the suppression zone and a higher vergence value is obtained 125 However it

is important to note that the significance of such suppression in binocular individuals is unknown 126 Again in special cases such as suppression large abnormal findings (compared to

normative ranges) may permit better functions in reading than a lower abnormal finding 183 For

example an individual who totally suppresses the vision of one eye is less apt to have difficulty

at reading than the individual who only partially suppresses the vision of one eye 183

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups The

mean fusional convergence capacities at distance were 1680 prism dopters for both the control

and the dyslexic group At near the corresponding figures were 2640 prism diopters and 2670

prism diopters respectively The mean fusional divergence capacity at distance was 650 prism

diopeters and 620 prism diopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 1050 and 1020 prism diopters respectively There was no

statistically significant difference between the groups

Latvala et al 3 reported a prevalence of fusional amplitude greater than or equal to 32 prism

diopters to be 61 for the control group and 75 for the dyslexic group using referral criteria

of 32 prism diopters at a distance of 33cm At near a testing distance of 40cm was used in this

study The difference in prevalence between the two studies might be from the different test

distances used Furthermore as detailed by Rouse et al 158 large intra subject intra-examiner

and inter-examiner variations in the assessment of fusional reserves have been reported

Vergence amplitudes have been reported to vary with alertness that is whether the subject is

tired or rested or under the influence of a toxic agent 184

129

Bedwell et al 171 reported that neither convergence nor divergence were significantly related to

reading difficultyrdquo Similarly Metsing and Ferreira103 found no statistically significant

difference between fusional vergences amplitude (Base-out base-in) at near No details were

provided

There are several limitations with the assessment of fusional reserves Sheedy (cited by Rouse et

al) 158 remarked that ldquo the general opinion is that when fusional vergence tests are repeated on

the same patient the second value found may be quite different from the first a difference of 10

PD from one fusional vergence amplitude measurement to another is not unusual unless rigorous

controls ar a typical difference of 3- 4 prism diopters but can measure differences as large as 12

prism diopter on follow up visit and that inter-examiner variation can be as large as 10 to 16

prism diopters The authors concluded that the positive fusional vergences difference could be

due to children having difficulty understanding the instructions or expected endpoints children

being slower responders or may be poorer observers The blur readings on the vergence reserves

for both distance and near were excluded in the analysis as it was difficult to get the children to

elicit a proper response It is possible that the children did not understand the instruction or the

concept of blur despite repeated explanations and trial readings This variable is often difficult

for young subjects to understand Similar problems were encountered by Evans et al 100 who

commented that several subjects were unable to appreciate a blurrdquo Scheiman et al 126 stated

that ldquowe were unable to consistently obtain a blur finding from our samplerdquo and that rdquo less

than13 of the subjects were able to report a blur we therefore only recorded the break and

recovery findingsrdquo

Wesson et al125 emphasized that the ldquomajor limitations of any vergence measurement

isrdquohellipnecessity to obtain a subjective response to blur break and recovery and that working with

children whose responses are not reliable is difficultrdquo It was suggested that a break and recovery

point could be determined by observing the subjects eye movements as prism power is increased

This technique was applied in this study However most children have difficulty maintaining

fixation long enough to measure vergence ranges

Vergence is influenced by several factors including awareness of the distance of the object

130

(proximal vergence)cross-linking with the accommodative system (acommodative vergence) and

the fine tuning of ocular alignment during the fusion of each monocular image into a single

percept (fusional vergence) 161 and other factors such as test target and lighting conditions 125

Generally in the assessment of a patientrsquos binocular status the fusional reserves may be more

useful when analyzed with the phoria measurements 115

The vergence system is closely related to the accommodative system and symptoms may at times

appear similar The symptoms associated with deficiencies with the vergence system include

letters or words appear to float or move around postural changes noted when working at a desk

difficulty aligning columns of numbers intermittent diplopia at either distance or near 185

Furthermore ldquodysfunctional vergence could cause an excess of eye movements especially of

small saccades The ability to continue the rapid decoding of the visual characters decreases as

more stress is placed on the vergence mechanism of the dyslexic during reading which makes

them tire more quickly than normal readersrdquo 6

A possible explanation for the relationship between vergence control and reading proposed by

Riddel et al (cited by Morad et al) 186 is that children with poor vergence control had impaired

accuracy of spatial localization that may impend their ability to accurately determine the position

of letters within words

527 Ocular Pathology

Two participants from the dyslexic group had cataracts Only the study by Hoffamn 83 reported

that one child had conjunctivitis

54 Summary

The results of this study do not vary significantly from the international studies in spite of the

differences in study designs tools and sample sizes Although studies have indicated differences

131

in the prevalance of open angle glaucoma and myopia between Caucasian and African

populations this study does not indicate a specific difference in the prevalance of vision variables

between Caucasian and African dyslexic school children However the lack of studies on

African children makes it difficult to reach any conclusions regarding structural differences that

may affect the presence of dyslexia Comparing the findings of this study with those done

internationally highlights the complexity of investigating the visual conditions that may be

associated with dyslexia and the need for rigorous and consistent testing methods in order to be

able to compare results

Having completed a discussion of the study findings in Chapter Five Chapter Six will present the

study conclusion and indicate its limitation as well as recommendations for future studies

132

CHAPTER SIX CONCLUSION

61 Introduction

Being able to read and write is an essential part of being able to interact with the world As

vision plays a major role in reading and the learning process a normally working visual system is

essential for efficient reading

With as many as 20 of Caucasian children being affected to a greater or lesser degree by

dyslexia 1631 there is a need to identify its cause and develop remedial actions to reduce its

effects As studies 4 59 have shown that intellectual ability is not the cause of dyslexia research

to enable people to reach their full potential in spite of their impaired reading ability requires a

multidisciplinary approach of which optometrists are part

This study aimed to determine the prevalence of vision conditions in a South African population

of African dyslexic children and to study the relationship between vision and dyslexia in an

African setting This was done by investigating visual acuity refraction and binocular functions

between two groups of 31 African school children one group attending a school for children with

learning difficulties and the other a mainstream school This enabled a study of possible vision

defects in African dyslexic children as well as a comparison between the two groups

The study targeted African school children its objectives being to

1 determine the distribution of visual acuity disorders among dyslexic children

2 determine the distribution of refractive errors among dyslexic children

3 determine the distribution of heterophorias among dyslexic children

4 determine the distribution of strabismus among dyslexic children

5 determine the distribution of accommodation disorders among dyslexic children

6 determine the distribution of vergence disorders among dyslexic children

7 determine the distribution of ocular pathology among dyslexic population

133

8 compare these findings to a similar group of non-dyslexic children

A review of the literature showed that most studies that investigated visual functions in dyslexic

school children were conducted on Caucasian population and that the findings of these studies

were inconclusive The broad areas of vision investigated in the previous studies 2-6 87-89 were

visual acuity and refraction binocular vision and ocular pathology

As studies conducted on the African population was lacking this study examined visual acuity

(using LogMar charts) refraction (static retinoscopy without cycloplegia) near point of

convergence (RAF rule) [accommodation functions 2 flipper lenses to assess accommodation

facility MEM retinoscopy for accommodation posture and pushndashup method to evaluate

amplitude of accommodation] ocular alignment (cover test and Maddox Wing) and strabismus

(Hirschberg test) and fusional reserves (using prism bars)

The study was conducted on a African population of 31 dyslexic school children selected from a

school for children with learning difficulties and 31 control participants from a mainstream

school in Durban Their ages ranged between 10-15 years The participants were selected using

the convenient sampling method as there were only a few participants classified as being

dyslexics All data collection procedures were conducted at the respective schools

The prevalence of vision conditions were presented in percentages () Data was analyzed using

the Statistical Package for Social Sciences (SPSS) and was expressed as mean (M) and standard

deviation (SD) and calculated for descriptive purposes at 95 confidence interval (CI) The level

of significance considered to support a hypothesis was taken as P lt 005 For comparison all

data from both groups was subjected to a two sample t test (2-tailed)

134

62 Summary of findingsIf an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants (control group) would be

expected

621 Visual Acuity

The prevalence of visual acuity worse than 69 (23 dyslexic and 225 control) is the same in

both the dyslexic and the control group and there was no statistically significant difference (p =029 023 right and left eye respectively This is similar to the findings by Buzzelli 6 and

Goulandris et al 90 which found no statistically significant difference between the two groups

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexia and the control group (p=029 for the right eye and 023 for the left eye)

622 Refractive Errors

The prevalence of refractive errors was similar in both groups There was no statistically

significant difference between the two groups (p=066 for the right eye and 092 for the left eye)

This is similar to the findings by Evans et al 2 which found no statistically significant difference

between the two groups (p=058)

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 066 for the right eye and 092 for the left eye)

a Hyperopia Hyperopia was more prevalent in the dyslexic group than in the control group

This is consistent with findings in other studies 88 93 95

b Myopia The prevalence of myopia was the same in both groups This is similar to the

findings by Eames 95 which found no difference in the prevalence of myopia between the

two groups

135

c Astigmatism Astigmatism was more prevalent in the control group than in the dyslexic

group This is similar to the findings by Alvarez et al 88 which found a higher prevalence of

astigmatism in the control group than in the dyslexic group

d Anisometropia The prevalence of anisometropia was the same in both groups (2 subjects

each) This is comparable to results reported by Latvala et al 3 where the prevalence of

anisometropia was 36 (2 subjects) for the dyslexic group and 6 (3 subjects) for the

control group

e Amblyopia Amblyopia was more prevalent in the dyslexic group compared to the control

group This is similar to the findings by Latvala et al 3 and Rosner and Rosner 93 which

found a higher prevalence of astigmatism in the dyslexic than in the control group

623 Near Point of Convergence

The prevalence of remote NPC (ge10cm) was higher in the control group than in the dyslexic

group and there was a statistically significant difference between the two groups (p= 0049) This

is similar to the findings by Kapoula et al 87 Latvala et al 3 and Evans et al 91 who found a

statistically significant difference between the dyslexic and the control groups on the NPC

functions

The null hypothesis was rejected as there was a statistically significant difference between the

dyslexic and control groups (p = 0049)

624 Heterophoria

The prevalence of exophoria at near was higher in the dyslexic group (95) than in the control

group with no exophoria There was no statistically significant difference between the two

groups (p= 059) The prevalence of esophoria in the dyslexic group was higher than that found

136

in the control group and there was no statistically significant difference (p =046) This is similar

to findings in other studies 3 12101163 which found no statistically significant difference between

the two groups

The null hypothesis was accepted as there was no statistically significant difference between the

dyslexic and control groups (p=059 for exophoria) and (p=046 for esophoria)

625 Accommodation Functions

a Amplitude of Accommodation The dyslexic group had a reduced monocular amplitude

of accommodation compared to the control group but there was no statistically significant

difference between the two groups (right eye p = 007 left eye p=022) This is similar

to the findings reported in other studies 3 87 91102 which found no statistically significant

difference between the two groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation amplitude (right eye

p=007 left eye p=022)

b Accommodation Facility The participants from the dyslexic group performed

significantly worse than the control group in accommodative facility function and there

was a significant difference between the two groups (p=0027) This result corroborates

findings reported in other studies 12 88 98100 which found a statistically significant

difference between the two groups

The null hypothesis was rejected as there was a statistically significant difference between

the dyslexic and control group in accommodation facility (p=0027)

c Accommodation Lag The prevalence of lag of accommodation was higher in the control

group compared to the dyslexic group and there was no statistically significant difference

There were no statistically significant difference between the two groups (right eye p=083

137

and left eye p= 060) This is similar to the findings by Evans et al 100 which found no

statistically significant difference between the two groups The mean accommodation lag in

the present study were 092D for control and 088D for the dyslexic which is comparable to

reports by Evans et al 91 who reported a mean accommodation lag of + 112D

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation posture

d Relative Accommodation (PRA NRA) The mean values for the relative

accommodation are similar in both groups and there was no statistically significant

difference between the groups (p=068 for NRA and p =051 for PRA) This is similar

to the findings reported Alvarez and Puell 88 which found no statistically significant

difference between the two groups in relative accommodation

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups in relative accommodation

626 Fusional Reserves

For the present study at near the negative fusional vergence (base-in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base-out

vergences) was similar for both groups There was no statistically significant difference between

the groups (p= 021 for BI break at near) and (p = 084 for BO break at near) This is similar to

the findings by Evans et al100 which found no statistically significant difference between the two

groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 021 for BI break at near) and (p = 084 for BO break at near)

627 Ocular Pathology

138

Two participants from the dyslexic group had cataracts

63 Implications of the Study Findings

a Causal Relationship This study presented the prevalence of some vision defects in a

South African African population of dyslexic school children as compared to a population

of non-dyslexic children and could not establish any causal relationship between vision and

dyslexia The vision parameters that showed that statistical significance may only be

reflective of factors associated with dyslexia but in a non-causal way This may mean that

the prevalence of the particular visual variable is much higher in the dyslexic population

than in the control group Vision is essential for reading but the vision anomalies that are

found to be associated with dyslexia also occur in children who do not have reading

difficulties It is therefore possible that other complex vision functions may be more related

to the etiology of dyslexia than peripheral visual factors such as NPC and accommodative

facility which showed statistically significant differences in the present study

b Risk Identification The findings of this study also suggest that dyslexic children are not

at more risk for the identified vision condition compared to non-dyslexic children

However even when vision defects may not be the cause of dyslexia the presence of

uncompensated vision defect in a dyslexic child may constitute difficulty while reading

and should be compensated for Improvement in vision will make reading easier for the

dyslexic child

64 The Study Strengths

The study methodology was based on reports found in other studies and consisted of the

following

a Classification The subjects selected for the dyslexic group represent a dyslexic population

that was classified according to the criteria used in the study by Latvala et al 3 and Evans et

139

al 100 The study was undertaken at a school for learning disabled learners and all students

were assessed and categorized as being dyslexic by the psychologists

b Study Examiner Examination of the subjects was conducted by the same optometrist who

had some experience in paediatric optometry This minimised the possibility of bias and

inter-examiner variability

c Examination Techniques The examination techniques used in this study were the same

as the techniques used in major referenced studies which were published in peer-reviewed

journals

d International Relevance It is the first study that has assessed broad vision functions in

a dyslexic population in an African setting

6 5 Limitations of the Study

The factors which may limit the generalization of findings of the study are as follows

a Inattentiveness This occurred in some children after numerous tests and the possibilities

of their lack of understanding of certain instructions However this problem was

minimized as trial readings were taken for some procedures and the participants were

instructed accordingly As stated in Section 34 testing was discontinued when the child

was tired This approach helped to minimize fatigue which could have negatively

affected the results In addition objective techniques that required minimal responses

from the participants were utilized in several procedures such as the cover test

retinoscopy and the monocular estimation method for evaluating accommodation posture

b Convenience Sampling This method of sampling was used given the limited number of

available subjects from the target population The available studies 235687-91102103 on

the subject of dyslexia learning disabilities and vision also used the convenience

sampling method

140

c Generalised Findings The generalization of the findings of this study is limited by the

fact that the refraction data was collected without the use of cycloplegia At the inception

of the data collection the diagnostic drug usage by optometrists was not approved in the

scope of practice in South Africa

d Sample Size The sample size was small as there was only one school for dyslexic

African children in Durban at the time of this study However review of the literature

also reveals that the sample sizes on studies conducted on dyslexia learning disabilities

and vision were typically small The targeted sample size of one hundred for this study

could not be met due to the limited number of subjects available The average sample size

of eight published studies 356 8790100 102 conducted on dyslexic children was 41

e Suppression Control The assessment of binocular accommodation facility and the

vergence reserves were performed without controlling for suppression

f Assessment of Relative Accommodation the use of a Phoropter would have yielded a

more conclusive result

g Eye Movements these were not assessed

Despite the acknowledged limitations inherent in this study the study provides useful

information and a research perspective on the prevalence of vision defects in a South African

population of dyslexic children which has not been conducted before

66 Recommendations

The following recommendations are made based on the findings of this study

661 Future Research

141

1 The same study with a similar protocol larger sample size with randomized sampling

2 Investigate the relative accommodation in in both groups using the phoropter

3 Assess the binocular functions with suppression control

4 Assess refractive error under cycloplgia

662 General Recommendations

1 All dyslexic children should have their eyes examined routinely to rule out

possibility of vision defects impacting on their reading performance

2 The Department of Education be reminded that visual defects can impact negatively on

the reading ability of learners and that teachers need to be more aware of pupils who may

present with signs of visual problems such as holding book too close or battling to see

the chalk board

3 The principals of the mainstream school should also be informed that vision defects were

detected in the non-dyslexic children

67 Conclusion

As a neurological condition which manifests primarily as a language-based disorder with

difficulty with words dyslexia requires a multi-disciplinary approach to its management with

an eye examination being needed to eliminate any possible effects that ocular conditions may

contribute to reading problems

The study provided the prevalence of disorders of visual acuity and refractive errors near point

of convergence accommodation dysfunction heterophoria strabismus and fusional reserves

among African dyslexic children of age range 10-15 years The only vision variable that was

more prevalent and that is significantly associated with dyslexia was the binocular

accommodation facility while only the near point of convergence (break and recovery) was

significantly more prevalent in the control than the dyslexic group The existence of these

statistically significant differences between the two groups may not imply clinical relevance

due to the small sample size The comparison of vision characteristics between the dyslexic

142

and control group indicates that the dyslexic children are not more at risk of having these

vision condition than the non-dyslexic children although sample size was small

Further research on African children will add to the body of knowledge about this group with

respect to the relationship between vision conditions and dyslexia about which very little is

known Only a few studies have provided evidence of the extent of dyslexia among African

children yet even a small percentage could result in many millions of children being affected

given the population of Africa While the problems that result from dyslexia are not life

threatening they do affect peoplersquos opportunities and quality of life

Unfortunately many African countries do not have the resources to provide specialized

schools for children with learning disabilities and every effort therefore needs to be made to

correctly identify the problems and possible causes of dyslexia In order for African countries

to implement the strategies to meet their millennium development education goals of ensuring

that by 2015 all children will be able to complete a full course of primary schooling they

need to provide not only for children in mainstream schools They need also to plan for

children whose academic ability would be enhanced by the appropriate diagnosis of learning

difficulties the provision of a simple eye test and spectacles or corrective apparatus where

appropriate

Dyslexia is not a disease for which a cure can be found rather long-term sustainable

interventions need to be put in place to ensure that the problems are identified and appropriate

remedial action is taken to minimize the impact it has on their life While many African

countries may not be able to afford schools for children with learning disabilities greater

awareness of the manifestation of the condition needs to be made to all teachers in an effort to

provide these children with the possibility of staying in main stream schools for as long as

possible where there are no alternatives With this in mind it is anticipated that this study will

contribute to the body of knowledge on vision conditions of African dyslexic school children

143

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3 Latvala ML Korhoenen TT Penttinen M Ophthalmic Findings in Dyslexic School Children Br J Ophthal 1994 78 339-343

4 Aasved H Ophthalmological Status of School Children with Dyslexia Eye 1987 161-68

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7 Naidoo KS Africa Vision Research Institute Monograph 2007

8 Wu SY Nemesure B Leske MC Refractive Errors in a African Adult Population The Barbados Eye Study Invest Ophthal amp Vis Sci 1999 40 (10) 2179-2184

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11 Raju D 1997 Poor Oculomotor Control in Dyslexia Masters Degree Thesis University of Durban-Westville

12 Keily PM Crewther SG Crewther DP Is there an Association between Functional Vision and Learning Clin amp Exp Optom 2001 84 (6) 346-353

13 Krupska M Klein C 1995 Demystifying Dyslexia London Language and Literacy Unit

14 Schumacher J Hoffmann P Schma C Schulte G Genetics of Dyslexia The Evolving Landscape J Med Genet 2007 44 289ndash297

144

15 Pushpa S Nallur B Biological basis of Dyslexia A Maturing Perspective Current Science 2006 90 (2) 168-175

16 Sherman G International Dyslexia Association Fact Sheet 969 January 2000

17 Berninger V Dyslexia The Invisible Treatable Disorder Learning Disability Quarterly2000 23 (3) 125

18 Harrie RP Weller C Dyslexia httpwwwericecorg Last accessed January 2007

19 Critchley M Is Developmental Dyslexia the Expression of a Minor Cerebral Damage The William Copeland Memorial Lecture National Hospital London 1966

20 Christenson GN Griffin JR Wesson MD Optometrys Role in Learning Disabilities Resolving the Controversy J of Am Optom Assoc 1990 61 (5) 363-372

21 Evans B 2007 Dyslexia and Vision 2nd Edition Whirr Publishers London

22 Flax N Visual function in Dyslexia Am J Optom Arch Acad Optom 1968 45 574-87

23 Optometric Clinical Practice Guidelines Care of the Patient with Learning-Related Vision Problems America Optometric Association 2000

24 Tan L Spinks JA Eden GF Perfetti CA Siok WT Reading depends on Writing in Chinese Pro Natl Acad Sc 2005 102 (24) 8781 ndash 8785

25 httpfacultywashingtonedu What Is Dyslexia Last Accessed 15th September 2008

26 Shaywitz S Overcoming Dyslexia A new and complete science-based program for reading problems at any level 2003 New York NY Alfred A Knopf

27 Holland KC A look at the Reading Process Optician July 3rd 1987 11-13

28 Bedwell CH The Visual and Ocular aspects of Reading Difficulty Optician Sept 24th 1982 26-38

29 Rayner Keith Eye Movement in Reading Perceptual and Cognitive ProcessesCanadian Psychology Feb 1995

30 Sibylina M Brochure from Learning Disabilities Council of America 1998

31 Pavlidis GT Eye Movements The Diagnostic Key to Dyslexia S Afr OptomJuneAugust 1988 29-39

32 Evans B The Role of the Optometrist in Dyslexia Part 1 Specific Learning Difficulties Optometry Today 2004 January 30 29-33

vi

(p=0049) and recovery (p=0046) The prevalence of poor binocular accommodation

facility at near was higher in the dyslexic group than in the control group and there was a

statistically significant difference between the two groups (p = 0027)

Vision defects such as hyperopia astigmatism accommodation lag convergence

insufficiency poor near point of convergence and accommodative infacility were present in

the dyslexic pupils but they were no more at risk of any particular vision condition than the

control group This study provided the prevalence of vision conditions in a population of

African dyslexic children in South Africa the only vision variable that was significantly

more prevalent in the dyslexic population being the binocular accommodation facility at

near although the study was unable to find a relationship between dyslexia and vision The

statistically significant difference may not imply clinical significance due to the small

sample size However it is recommended that any vision defects detected should be

appropriately compensated for as defective vision can make reading more difficult for the

dyslexic child

The sample size may have been a limitation however this was comparable with studies

reviewed most of which had sample sizes of less than 41 Due to the range of possible

ocular conditions that could affect dyslexia it is recommended that a larger sample size be

used to ensure more conclusive results Testing for relative accommodation with a

phoropter would provide more accurate results and accommodation facility and fusional

reserves would be better assessed with suppression control The study provides information

and an indication of research needs regarding the prevalence of vision defects in an African

South African population of dyslexic children

vii

TABLE OF CONTENTS

CONTENTS PAGETITLE PAGE iDECLARATION iiACKNOWLEDGEMENTS iiiABSTRACT ivTABLE OF CONTENTS viiLIST OF APPENDICES xiLIST OF TABLES xiiLIST OF FIGURES xiiiLIST OF PUBLICATIONS FROM THIS THESIS xiv

CHAPTER ONE INTRODUCTION 1

11 Background and Rationale for the Study 112 Aim 213 Objective 214 Research Questions 215 Significance of study 316 Goals of Study 317 Null Hypothesis 318 Research (alternative) Hypothesis 419 Scope of Study 4110 Assumptions 5111 Study Outline 5112 An overview of Dyslexia 5113 The Reading Process 8114 Eye Movement in reading 9115 Characteristics Features of Children with Dyslexia 10116 Implications of Dyslexia for learning 11

1161 Reading and Comprehension 121162 Handwriting 121163 Proof-reading 12

117 Prevalence of Dyslexia 12118 Aetiology of Dyslexia 13

1181 Neurological Factors 141182 Genetic Factors 151183 Peri-natal Factors 151184 Substance Use during Pregnancy 151185 Environmental Factors 161186 Nutrition 16

119 Diagnosis of Dyslexia 171191 Exclusionary Method 17

viii

1192 Indirect Method 171193 Direct Method 18

120 Functional Imaging Techniques used in the Diagnosis and Study of Dyslexia 191201 Positron Emission Tomgraphy (PET) 191202 Functional Magnet Resonance Imaging (fMRI) 19

121 Classification of Dyslexia 201211 Primary Dyslexia 211212 Secondary Dyslexia 21

122 Sub-types of Dyslexia 211221 Dysphonetic (auditoryphonetic) 211222 Dyseidetic (visual-spatial) 211223 Dysnemkinesia (motor) 211224 Dysphonetic-Dyseidetic 23

123 Theories of Dyslexia 231231 Phonological-Deficit Theory 231232 Cerebellar Theory 251233 Visual Deficit Theory 261234 Auditory Processing Theory 27

124 Cultural Differences in Dyslexia 28125 Treatment Intervention 30126 Role of Optometry in the Multidisciplinary Management of Dyslexia 30

1261 Case History 311262 Refractive and Binocluar Functions 311263 Visual-Motor Skills 311264 Visual-Spatial Analysis 321265 Short-term Visual Memory (visual-motor recall) 321266 Auditory Analysis Skills 32

127 Optometric Treatment Options 331271 Correction of Vision Defects 331272 The use of Coloured Overlay 33

128 Potential Mechanism for Benefit from Tinted Lenses 35129 Summary of Chapter One 37

CHAPTER TWO LITERATURE REVIEW 39

21 Introduction 3922 Visual Acuity 4023 Refractive Errors 47

231 Myopia 47232 Hyperopia 49233 Astigmatism 49234 Amblyopia 50235 Anisometropia 50

24 Near point of Convergence 53

ix

25 Heterophoria 5526 Strabismus 5827 Accommodation Functions 59

271 Amplitude of Accommodation 59272 Accommodation Facility 61273 Relative Accommodation 64274 Accommodation Posture 64

28 Vergence Facility 6529 Fusional Reserves 66210 Ocular Pathology 68211 Summary of Chapter Two 73

CHAPTER THREE METHODOLOGY 75

31 Introduction 7532 Research Design 75

321 Sampling Design 75322 Study population and Participants 75323 Inclusion Criteria for Dyslexic Group 76324 Exclusion Criteria for Dyslexic Group 76

33 Ethical Clearance 7734 Testing Protocol 7735 Study Setting 7736 Data Collection Technique 78

361 Visual Acuity 79362 Refractive Error 79363 Near Point of Convergence 80364 Strabismus 80365 Heterophoria 81366 Evaluation of Accommodation Functions 83367 Fusional Reserves 85368 Ocular Health 87369 Vergence Facility 87

37 Statistical Analysis 8738 Summary of Chapter Three 89

CHAPTER FOUR RESULTS 91

41 Introduction 9142 Prevalence of Vision Defects 93

421 Visual Acuity 91422 Refractive Errors 92423 Near Point of Convergence 94424 Ocular Health 96

x

425 Heterophoria 96426 Accommodation Functions 97427 Fusional Reserves 101

43 Summary of Chapter Four 105

CHAPTER FIVE DISCUSSION 107

51 Introduction 10752 Prevalence of Vision Defects 107

521 Visual Acuity 109522 Refractive Errors 116523 Heterophoria 116524 Near point of Convergence 118525 Accommodation Functions 120526 Fusional Reserves 126527 Ocular Pathology 130

53 Summary of Chapter Five 130

CHAPTER SIX CONCLUSION 132

61 Introduction 13262 Summary of Findings 134

621 Visual Acuity 134622 Refractive Errors 134623 Near Point of Convergence 135624 Heterophoria 135 625 Accommodation Functions 136 626 Fusional Reserves 137627 Ocular Pathology 137

63 Implications of the Study Findings 138 64 The Study Strengths 138 65 Limitations of the Study 139 66 Recommendations 140

661 Future Research 140662 General Recommendations 140

67 Conclusion 141

REFERENCES 143

xi

LIST OF APPENDICES PAGE

APPENDIX A Clarification of Terms 156APPENDIX B Request for Consent from Department of EducationAPPENDIX C Letter of Consent from Department of EducationAPPENDIX D Letter of Consent from Addington Primary schoolAPPENDIX E Visual Acuity Conversion Table

xii

LIST OF TABLES

TABLE TITLE PAGE

11 Characteristic Features of Dyslexia 1012 Features of the Magnocelular and Parvocellular systems 2721 Conclusions from Major Studies on Dyslexia and Vision 7031 Data Collection Tools and Measurement Outcomes 7832 The Diagnostic Criteria for each Test Variable 8741 Descriptive Statistics for Visual Acuity 9242 Descriptive Statistics for Refractive Error 9443 Descriptive Statistics for Near Point of Convergence Break

and Recovery Points 9544 Descriptive Statistics for Heterophoria 9745 Descriptive Statistics for Accommodative Posture 9846 Descriptive Statistics for Binocular Accommodation Facility 9947 Descriptive Statistics for Amplitude of Accommodation 10048 Descriptive Statistics for Relative Accommodation 10149 Descriptive Statistics for Base-in Vergence Reserves at Distance 102410 Descriptive Statistics for Base-in Vergence Reserves for Near 102411 Descriptive Statistics for Base-out Vergence Reserves at Distance 103412 Descriptive Statistics for Base-out Vergences at Near 103413 Descriptive Statistics for Dyslexic and Control Groups

for all Variables 104

xiii

LIST OF FIGURES

FIGURE TITLE PAGE

41 Prevalence of Visual Acuity 9242 Prevalence of Refractive Errors (total) 9343 Prevalence of the Types of Refractive Errors 9444 Prevalence of NPC Break 9645 Prevalence of Phoria at Near 9646 Prevalence of Accommodation Lag 9847 Prevalence of Reduced Binocular Accommodation Facility at Near 9948 Prevalence of Relative Accommodation 101

1

CHAPTER ONE INTRODUCTION

11 Background and Rationale for the Study

Optometry has long been involved in the subject of vision and learning As primary eye care

practitioners optometrists receive referrals from parents teachers psychologists and other

professionals who usually seek the optometristrsquos advice about whether a childrsquos vision problem

may be contributing to or is responsible for poor academic performance

According to Scheiman and Rouse1 learning which is the acquisition of knowledge or skill can

be achieved through study experience or instruction and that between 75 and 90 of what a

child learns is mediated through the visual pathways As vision plays a major role in reading and

the learning process any defects in the visual pathway disrupt the childrsquos learning

ldquoThe three interrelated areas of visual functions are 1

1 Visual pathway integrity including eye health visual acuity and refractive status

2 Visual efficiency including accommodation binocular vision and eye movements

3 Visual information processing including identification and discrimination

spatial awareness and integration with other senses

Several studies2-6 have been conducted to investigate the relationship between vision and dyslexia

on Caucasian populations while very few have been done on African subjects Consequently

most inferences and conclusions on dyslexia are based on research conducted in Caucasian

populations Naidoo7 reported that inferences drawn from studies in Caucasian populations may

not be appropriate for the African population as the lsquoAfrican Eyersquo differs from the Caucasian eye

For example epidemiological studies have reported variations in refractive errors where the

prevalence of myopia is higher among Caucasian persons than in African persons8 9 while

African populations had higher open angle glaucoma prevalence than Caucasian persons10

This study attempts to contribute to the literature by examining the prevalence of a broader

spectrum of vision variables in dyslexic school children in order to investigate a possible

2

relationship between dyslexia and vision in African school children in South Africa A search of

the literature revealed that very little research has been conducted in Africa the only one being

conducted in 1997 by Raju11 Raju11 investigated oculomotor functions in dyslexic school

children in South Africa and reported that 65 of the normal readers exhibited normal

oculomotor control as compared to 11 of the dyslexics Furthermore 54 of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills while 12 of the

controls displayed the same

12 Aim

The aim of the study was to determine the prevalence of vision conditions in a South African

population of African dyslexic children and to study the relationship between vision and dyslexia

13 Objectives

1 To determine the distribution of visual acuity disorders among African dyslexic children

2 To determine the distribution of refractive errors among dyslexic children

3 To determine the distribution of phorias among dyslexic children

4 To determine the distribution of strabismus among dyslexic children

5 To determine the distribution of accommodation disorders among dyslexic children

6 To determine the distribution of vergence disorders among dyslexic children

7 To determine the distribution of ocular pathology among dyslexic population

8 To compare these findings to a similar group of non-dyslexic children

14 Research Questions

1 What is the prevalence of vision conditions among African dyslexic children of age range

10 and 15 years

3

2 What is the relationship between dyslexia and vision

15 Significance of the Study

The study will attempt to

1 Identify high-risk ocular conditions among dyslexic children which might influence the

need for certain examinations or that could affect the childs ability to learn to read

2 Relate findings of this research work to that of previous researchers

3 Add to the knowledge base in the area of vision and dyslexia

16 Goals of Study

1 To obtain quantitative information on the prevalence of visual conditions in dyslexic

children in the African population

2 To obtain information on the relationship between vision disorders and dyslexia

3 To enable improvement of eye care for children with dyslexia

17 Null Hypothesis

1 There will be no statistically significant difference between the mean visual acuity of the

dyslexic group and of the control group

2 There will be no statistically significant difference between the mean refractive errors of the

dyslexic group and of the control group

3 There will be no statistically significant difference between the mean phoria in the dyslexic

group and of the control group

4 There will be no statistically significant difference between the strabismic mean angle of

deviation of the dyslexic group and of the control group

5 There will be no statistically significant difference between the mean accommodation

functions (amplitude facility posture) in the dyslexic group and of the control group

4

6 There will be no statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

1 8 Research (alternative) Hypothesis

1 There will be a statistically significant difference between the mean refractive errors in the

dyslexic group and of the control group

2 There will be a statistically significant relationship between the mean visual acuity of the

dyslexic group and of the control group

3 There will be a statistically significant difference between the mean of phoria of the

dyslexic group and of the control group

4 There will be a statistically significant difference between the strabismus mean angle of

deviation of the dyslexic group and of the control group

5 There will be a statistically significant difference between the mean accommodation

functions of the dyslexic group and of the control group

6 There will be a statistically significant difference between the mean fusional vergence

amplitude and reserves of the dyslexic group and of the control group

19 Scope of Study

1 This study is limited to the assessment of visual functions in dyslexic and normal readers

The visual functions investigated include visual acuity refractive error ocular alignment

near point of convergence accommodation functions and vergence reserves The

assessment of eye movement was not part of the study

2 Screening for and diagnosing dyslexia was not part of this study The school

psychologistsrsquo reports were relied on in selecting subjects who qualify to participate in this

study The information was obtained from the learnersrsquo school files 3 The scope of vision parameters investigated is limited only to the prevalence and

relationship between these variables in dyslexic and control groups as outlined in the

hypothesis Correlation analysis of different variables was not part of this study

5

110 Assumptions

In this report dyslexia is used synonymously with ldquodevelopmental dyslexiardquo ldquoreading

disabilityrdquo and reading disorderrdquo and will be used to describe children who find it extremely

difficult to learn to read despite having normal intelligence appropriate educational opportunities

and absence of emotional disorders Children so defined have reading age that is two or more

years behind their chronological age12

111 Study Outline

This thesis is divided into six chapters Chapter one indicates the rationale of the study provides

an overview of dyslexia and details its purpose objectives significance and goals Chapter Two

(the literature review) gives a detailed review of literature in the area of dyslexia learning

disabilities and vision with particular emphasis to vision variables as outlined in the hypothesis

to be tested This is followed by methodology in Chapter Three which details the research

methodology in which a case-control study was done in two schools in Durban The areas of

vision variables evaluated include visual acuity and refraction binocular vision and ocular health

evaluation Chapter Four (Results) presents an analysis of the results obtained from the study

Chapter Five (Discussion) discusses the study findings and their meaning Chapter Six presents

conclusions from the study based on the study results indicating the implications applications

and recommendations for future study and the study limitations

112 An overview of dyslexia

Some children find it difficult to learn to read despite having normal intelligence appropriate

educational opportunities and absence of emotional disorders Their reading age is two or more

years behind their chronological age Such children are referred to as being dyslexics Where

there is an associated deficit in intelligence the condition may be described as generalized

learning disability12

6

The term dyslexia is derived from the Greek words dys meaning hard or difficult and lexia

from the word lexicos which means pertaining to words so dyslexia means difficulty with

words-either seen heard spoken or felt as in writing13

The World Federation of Neurologists13 and the International Classification of Diseases-1014

defined dyslexia as ldquoa disorder manifested by difficulty in learning to read despite conventional

instruction adequate intelligence and socio-cultural opportunity It is dependent upon

fundamental cognitive disabilities which are frequently constitutional in originrdquo

Dyslexia is a complex disability which affects different aspects of reading The acquisition of

reading related skills are coordinated by visual motor cognitive and language areas of the brain

Consequently dyslexia can result from deviation of normal anatomy and function of cognitive

and language areas in the brain 15

Dyslexia refers to a cluster of symptoms resulting in people having difficulties in specific

language skills particularly reading15 The mystery in dyslexia as a syndrome lies in the fact that

dyslexics have ldquodifficulty to learn to read despite having average or above average intelligence

adequate auditory and visual acuity absence of frank brain damage no primary emotional

problems and have adequate educational instructionsrdquo 16

Dyslexia is a developmental disorder that shows in different ways at different developmental

stages As documented by Krupska and Klein 13 dyslexia is a learning disability characterised by

problems in expressive or receptive oral or written language that manifest initially as difficulty in

learning letters and letter sound association which is followed by difficulty in learning letters and

reading words accurately which consequently results in impaired reading rate and written

expression skills (handwriting spelling and compositional fluency) Some dyslexics find it

difficult to express themselves clearly or to fully understand what others mean when they speak

The effects of dyslexia reach well beyond the classroom as it can affect the persons self-image

It makes them feel dumb13 17 Dyslexia describes a different kind of mind often gifted and

productive that learns differently An unexpected gap exists between learning aptitude and

achievement in school 18

Many dyslexics are creative and have unusual talent in areas such as art athletics architecture

7

graphics electronics mechanics drama music or engineering 18 Some dyslexic learn best with

a multi-sensory delivery of language content which employ all pathways of learning at the same

time seeing hearing touching writing and speaking18 It is primarily not a problem of

defective vision although underlying vision deficit has been known to exaggerate the dyslexics

reading problems 18

According to Critchley19 ldquothe child otherwise normal as regards to intellectual status and

emotional stability finds an isolated difficulty in mastering the significance which lies behind

printed or written symbols and of necessity finds it extremely hard to align the verbal

components of speech (as emitted by the mouth and as received by the ear) with the empirical

characters which are set out on paper in one form or anotherrdquo Dyslexics are unable to acquire

written language skills through ordinary learning and teaching methods and often fail to progress

in education19

A dyslexic individual displays a characteristic pattern of decoding (recognising) and encoding

(spelling) difficulties with written words20 Interestingly dyslexics may not have overt

difficulties with spoken language yet do have marked difficulties with written languages21 The

difference between written and spoken language is that written language is seen while spoken

language is heard This fact may explain why vision has always been implicated on the topic of

dyslexia 21

Dyslexics experience difficulties with visual-verbal matching necessary to establish word

recognition ability Even when a clear single visual image is present the dyslexic may be unable

to decode the printed word in some cases This may be related to the fact that the difficulty that

dyslexics have is at a much more fundamental stage in the reading process so the role of vision

function is quite different in dyslexia than in other types of reading difficulty22 (which may be

peripheral such as hyperopia leading to difficulty in reading) The two main situations in which

the term dyslexia now commonly applies are when the reader has difficulty decoding words (that

is single word identification) and the second is encoding words (spelling) The second type is a

frequent presentation in optometric practice that is when the reader makes a significant number of

letter reversals errors (example b-d) letter transposition in word when reading or writing

(example sign for sing) or has left-right confusion 23

8

113 The Reading Process

Language development entails four fundamental and interactive abilities listening speaking

reading and writing while learning to read involves a complex system of skills relevant to visual

(the appearance of a word) orthographic (visual word form) phonological and semantic

(meaning of word and phrases) processing and a variety of behaviors such as letter naming letter

perception word recognition and comprehension each of which uses a different part of the brain

Reading is an interpretation of graphic symbols24-26

For some children the ability to break a word into its smaller parts a task crucial in reading is

extremely difficult Dyslexia is a localized problem one involving the sounds and not the

meaning of spoken language Speaking is natural while reading is different it must be learned

To be able to transform the printed letters on the page to words that have meaning the letters

must be connected to something that already has inherent meaning- the sounds of spoken

language In order to learn to read children must learn how to link the printed letters on the page

to the sounds of spoken language and understand that words are made up of sounds and that

letters represent these sounds or phonemes24- 26 In other words learning to read requires that the

child be able to break the word into the individual components of language represented by letters

and to be able to tell the difference between the individual phonemes that make up a word

Learning to read requires that the central principle behind the alphabet is understood that is

words are made of phonemes and in print phonemes are represented by letters Phonemes are

the shortest units of sound that can be uttered in a given language and that can be recognized as

being distinct from other sounds in the language For example the word cat has three phonemes

ca ah and ta Blended together the sounds form the word rsquocatrsquo24-26 When one speaks the

sounds are blended ttogether and are said one cat 24-26 To learn to read a child must learn that

there are three separate sounds This is difficult for children with dyslexia The inability to break

word into its parts is the main reason why children with dyslexia have trouble learning to

read24- 26

Furthermore dyslexic children find it difficult to bring the print to language (such as when asked

to read what they just wrote) For example a child can copy the letters w-a-srdquo correctly but

9

when asked what was written a child with dyslexia may reply saw The problem in this case

may not be related to vision but rather one of perceptual skills of what the child does with a word

on a page The brain mechanism of going from print to language is phonologically based 24-26

Reading acquisition builds on the childrsquos spoken language which is already well developed

before the start of formal schooling Once a written word is decoded phonologically its meaning

will become accessible via the existing phonology-to-semantics link in the oral language system

Thus the childrsquos awareness of the phonological structure of sound of speech plays a major role in

the development of reading ability 24-25

114 Eye Movement in Reading

For a person to be able to read a letter or word the eye has to first identify the written material

Normally the eyes scan across the line of words on the page stopping to fixate at various points

then making short jumps from one position to the next (some saccade eye movements are

involved) Typically there are around five fixation pauses in a line Sometimes the eye will

regress and go back to a previously passed over word element A reader views the text mainly

with the controlling or reference eye whilst the other eye diverges and converges within fine

elements27 A sophisticated mechanism27 (a connection of the visual receptors of the retina with

the visual nervous pathway to the brain) 28 ensures control of this drift and prevents diplopia from

being consciously observed These drifts and subsequent recovery can be clearly seen by a

number of techniques including infrared observation of the eyes27

The minimum time a person can spend on fixation and move to the next is around 250

milliseconds27 For skilled readers the average saccade length is 7-9 letter spaces and the

duration of saccades is of the order of 25-50 milliseconds29 while the average fixation pauses last

between 200-250 milliseconds27 29 Thus our eyes are still for some 90 of the time while

reading and are subject to large changes in acceleration and deceleration Even a good reader

will regress and go back 10 to 20 of the time27 29

The textual image seen by the eyes is imaged onto both maculae and transmitted to both cerebral

10

cortex where it is first received in the primary visual cortex (Brodmanrsquos area 17) with some

initial processing of colour and tone occur Progressively higher levels of processing occur in

adjacent areas 18 and 19 with the highest level of visual language processing taking place in the

angular gyrus (area 39) The angular gyrus is central to reading since it is here that hearing

speech and vision meets and the written word is perceived in its auditory form The auditory

form of the word is then processed for comprehension in Wernickersquos area as if the word had been

heard25 27

115 Characteristic SignsSymptoms of Children with Dyslexia

Dyslexia is heterogeneous in nature and there is no single pattern of difficulty that affects all

people Dyslexic children frequently show a combination of characteristics (Table 11)

Table 11 Characteristic SignsSymptoms of Dyslexia 16 30-31

Reading Visionminus Holds book too close

minus Word reversals

minus Skips complete words

minus Re-reads lines

minus Points to words

minus Word substitution

minus Sees double

minus Poor comprehension in oral reading

minus Might see text appearing to jump around on a page

minus Unable to tell difference between letters that look similar in shape such as o and e

minus Unable to tell difference between letters with similar shape but different

orientation such as b and p and q

minus Letters might look jumped up and out of order

minus Letters of some words might appear backwards eg bird looking like drib

11

Spelling

minus Omission of beginning or ending letters

minus Can spell better orally than written

minus Letter reversals

minus Wrong number of letters in words

Difficulty with Reading

minus Difficulty learning to read

minus Difficulty identifying or generating rhyming words or counting syllables in

words (Phonological awareness)

- Difficulty with manipulating sounds in words

minus Difficulty distinguishing different sounds in words (Auditory discrimination)

AuditoryVerbalminus Faulty pronunciation

minus Complains of ear problems

minus Unnatural pitch of voice

minus Difficulty acquiring vocabulary

minus Difficulty following directions

minus Confusion with beforeafter rightleft

minus Difficulty with word retrieval

minus Difficulty understanding concepts and relationships of words and sentences

116 Implications of Dyslexia for Learning

Dyslexia affects people in different ways and depends on the severity of the condition as well as

the effectiveness of instruction or remediation The core difficulty is with word recognition and

reading fluency spelling and writing Some dyslexics manage to learn early reading and spelling

tasks especially with excellent instruction but later experience their most debilitating problems

when more complex language skills are required such as grammar understanding textbook

material and writing essays The dyslexic syndrome therefore affects reading and learning in the

12

following ways263034

1161 Reading and Comprehension

One major difficulty dyslexics have is the time it takes to read and understand a text This makes

reading an extremely difficult task and they may report that they get tired and may get headaches

after reading for a short while30 There is also difficulty in assimilating what they have read even

when they know the words Multiple-choice questions are usually difficult for them as they find

it difficult to recognise words out of context263034

1162 Handwriting

Dyslexics have problems with handwriting since for some forming letters takes concentration

that their ideas get lost and their written expression suffers30

1163 Proof-reading

Dyslexics with visual processing difficulties usually have difficulty in identifying errors in

their own writings Reading difficulties make it uneasy for them to see their own errors in

expression and sentence structure grammar26 30

117 Prevalence of Dyslexia

Estimates of the prevalence of dyslexia in Caucasian populations vary from 5 to 20 in the

general population16 Pavlidis 31 reported that about 1 to 3 of the total population of children

suffers from dyslexia and that 20 to 30 of those cases classified as ldquogeneral reading failures

are probably dyslexics Evans32 reported a prevalence of 4 - 5 in children and stated probably

more boys than girls are dyslexic Park33 documented that up to 15 of the school-age

population has some degree of dyslexia with a ratio of four boys to one girl The large variability

in the prevalence reported by different authorities may be attributed to the differences in the

diagnostic criteria and the cut-off point applied to the psychometric tests14 Dyslexia is the most

common and the most researched type of learning disability34

13

A literature search revealed one study in Egypt that reported the prevalence of specific reading

difficulty in 2nd and 3rd grades in elementary school population to be 1 and the male to female

ratio to be 27 to 1 The authors concluded that the prevalence was low compared to that reported

in western countries and that the difference may be due to the way the Arab language is written

and read 35

Contrary to the above reports that males tend to be more dyslexic than females Guerin Griffin

Gottfried and Christenson 36 studied dyslexia subtypes and severity levels with respect to gender

differences The authors36 concluded that

when objective criteria such as the discrepancy between achievement (in

reading andor spelling) and intelligence or direct assessment of coding

skills are used as criterion and non-referred or random samples are

assessed the ratio of males to females identified as dyslexics approaches

11 When identification depends on referrals from parents and or

teachers more males than females may be detected because of the nature

and intensity of boysrsquo behavioural reactions to the disability

According to Shaywitz and Shaywitz34 good evidence based on sample surveys of randomly

selected population of children now indicates that dyslexia affects boys and girls comparably

118 Aetiology of dyslexia

The literature contains diverse theories (not less than four major theories) on the aetiology of

dyslexia probably due to the complex nature of the subject but several authors13-15 26 37-42 have

maintained that it is a neurological disorder with a genetic origin Despite decades of

multidisciplinary research neuropsychology brain anatomy and neuro-imaging 37-42 the specific

causes of dyslexia are still unknown The leading assumption however is that dyslexia

fundamentally stems from subtle disturbances in the brain and other possible explanations of the

cause of dyslexia such as language processing difficulties are based on the neurological etiology

14

The aetiological considerations in dyslexia are

1181 Neurological Factors

As documented by Habib 37 the concept of the neurological basis of dyslexia was first mentioned

independently by a Scottish ophthalmologist Hinshelwood in 1895 and a British physician

Morgan in 1896 Both found similarities of certain symptoms of dyslexia in some children

Dyslexia was then described as ldquovisual word blindnessrdquo The earlier understandings for the

neurological basis of dyslexia came from neuro-pathological studies of brains from dyslexic

persons According to Habib37 it was first reported by the French neurologist Dejerine in

1891 that damage to the left inferior parieto-occipital region (in adults) resulted in variable

degree of impairments in reading and writing suggesting that the left angular gyrus may play a

role in processing the ldquooptic images of letterrdquo It was subsequently thought that impaired reading

and writing in the young dyslexic patients could be due to abnormal development of the same

parietal region which was damaged in adult alexic patients 37

Another perspective 13 37 is that the dense distributions of ectopias (the presence of neural

elements in places where they are not supposed to be found) all over the cerebral cortex

(particularly in the perisylvian language areas) resulted in alteration of brain organisation One

such alteration in dyslexia is the lack of asymmetry in a language-related cortical region called

the planum temporale (an auditory area that lies on the superior surface of the temporal lobe) In

normal subjects the platinum temporale is usually larger in the left hemisphere However the

dyslexics showed symmetry of the planum temporale 13 37 Furthermore these ectopias may

reduce cortical connectivity as suggested by recent positron emission tomography and magnetic

resonance imaging studies 37

Another speculation was that the development of ectopias could be due to foetal hormonal

(possibly testosterone) imbalances during late pregnancy which could also account for the

possible male predominance in dyslexia 15 30 37 38

The maturation lag which is believed to be due to the slower development of the nerve fibres of

the corpus collosum (bundle of cells connecting the left and right hemisphere of the brain) has

also been theorized to be a cause of dyslexia A delay in the growth of the fibres linking the two

15

hemispheres would result in affected children being physiologically incapable of tasks which

require hemispheric organisation This leads to the inadequate development of the language

functions 12 37 However recent neuro-imaging studies 39-42 have shown that impaired reading of

alphabetic scripts (such as in the English language) is associated with dysfunction of the left

hemisphere posterior reading systems primarily in the left temporo-parietal brain regions

1182 Genetic Factors

Genetic considerations in dyslexia are derived from the fact that dyslexia runs most often in

families However it is speculated that different forms of dyslexia may occur within the same

family which also may means that it seems likely that the inheritance is indirect 15 31 However

genetic markers have been identified in chromosome 18 38 and chromosomes 6 and 15 15 In

fact chromosomes 2 (DYX3) 6 (DYX2) and 15 (DYX1) in dyslexics have been reported to be

inherited in an autosomal dominant mode of transmission 15 Furthermore Ramus 43 stated that

ldquogenetically driven focal cortical abnormalities such as ectopias and microgyri in specific areas

of left perisylvian cortex involved in phonological representations and processing are the

primary cause of dyslexia which is consistent with anatomical studies of dyslexic brains showing

loci of cortical abnormalities functional brain imaging studies showing that the very same areas

are involved in phonological processing and show abnormal activation in dyslexicshelliprdquo

1183 Peri-natal Factors

Peri-natal factors may manifest as problems during pregnancy or complications after delivery

More importantly it can also happen that the mothers immune system reacts to the foetus and

attacks it as if it was an infection It has also been theorized (based on partial evidence) that

dyslexia may occur more often in families who suffered from various autoimmune diseases (such

as rheumatoid arthritis Gravesrsquo diseases multiple sclerosis systemic lupus Sjogrenrsquos

syndrome)3137

1184 Substance Use during Pregnancy

16

Drugs such as cocaine (especially in its smokable form known as crack) seem to affect the

normal development of brain receptors of the foetus It has been documented that mothers who

abuse substance during pregnancy are prone to having smaller babies and such infants are

predisposed to a variety of problems including learning disorders Thirdly alcohol use during

pregnancy may influence the childrsquos development and lead to problems with learning attention

memory or problem-solving There is also a risk of foetal alcohol syndrome (characterised by low

birth weight intellectual impairment hyperactivity and certain physical defects) 30

1185 Environmental Factors

Environmental factors are risk factors for the development of reading disabilities 44 Reading

disabilities which are due to environmental influences have been referred to as non-specific

reading disabilities 20 or pseudo- dyslexia as the reading disorder is secondary to extraneous

factors and genetic factors play an important role 45 ldquoEnvironment factors play a significant role

in the level to which neurological impairments manifest themselves Encouraging nurturing and

stimulating surroundings can reduce the impact of risk factors on learning performance and life

skillsrdquo 44 Environmental circumstances which may profoundly influence the dyslexic child

include parental attitude the role of multilingualism in the home attention problems the

drawback of frequent changes of school the personality of the teacher and the childs emotional

reaction to his difficulties 152030

It seems that the influence of environmental factors may best be described as a potential risk

factor than a causative factor in dyslexia as there are diverse views on the relationship between

environmental influences and dyslexia According to Grigerenko46 socioeconomic level

educational opportunity and home literacy level are major environmental risks for the

manifestations of developmental dyslexia hellip the ldquomagnitude of the effects was not strong enough

to view them as powerful causal factorsrdquo

1186 Nutrition

Nutrition plays an important role in bridging the gap between genetic constitution and a childs

potential for optimal development Nutrition is a basic requirement for the maintenance of

17

growth and overall development while malnutrition and under-nutrition are associated with

disorders in the normal development of intelligence perceptual maturation and academic

achievements 47 Studies conducted by Grant Howard Davies and Chasty 48 showed that there

is an association between dyslexia and low concentration of zinc Zinc is necessary for normal

brain development and function and essential for neurotransmission44

119 Diagnosis of Dyslexia

The first stage in the diagnosis of dyslexia is with awareness by parents or educators that a

problem in reading exists The initial point of assessment starts with the medical practitioner

(possibly a paediatrician) who conducts a complete medical examination and obtains a

comprehensive health history A correct diagnosis of dyslexia starts with a very well taken case

history especially good awareness of the childrsquos developmental history This may give

indications of any medical condition that may be contributing to reading difficulties If indicated

the child may be referred for a neurological examination If dyslexia is suspected the physician

may then refer the child for further evaluation and treatment by a specialist in psycho-educational

assessments In most cases the diagnosis of dyslexia is usually performed by the psychologist

The Psychologists conduct psychometric assessments which include assessment of intelligence

quotient (IQ) various aspects of reading performance spelling mathematics sequential memory

and other aspects of cognitive and emotional development49 50 The major purpose of the

diagnostic process is to isolate the specific difficulties associated with dyslexia and to suggest

appropriate educational intervention 49 50

There are three methods for diagnosing dyslexia

1191 Exclusionary Method

In the exclusionary method the diagnosis of dyslexia is made when all factors involved in a

nonspecific reading disability (reading problems resulting from factors such as intellectual

disability dysfunction of hearing or vision inadequate learning experience socio-cultural

deprivation primary emotional problems poor motivation) are excluded 2051 The problem with

18

the exclusionary method of diagnosing dyslexia is that dyslexia cannot be diagnosed until after

the child has been failing in school for almost two years By this time constant failure may have

produced a negative attitude towards school and psychological problems 31

It provides a very

limited description of the features of the disorder Dyslexia is often defined in terms of the

discrepancy between reading achievement and chronological age or grade level failing to

consider the possible overlap between some of the intelligence quotient tests and reading tests 52

According to Snowling 53 IQ is not strongly related to reading and so the use of IQ in defining

dyslexia is inappropriate stating that many children with low IQ can read perfectly well and that

the discrepancy definition (based on the difference between reading achievement and

chronological age) of dyslexia cannot be used to identify younger children who are too young to

show a discrepancy

1192 Indirect Method

The second method of diagnosing dyslexia is the indirect approach A diagnosis is sought by

attempting to associate neurological soft signs such as finger ldquoagnosiardquo with reading failure The

drawback with this method is that many dyslexics do not manifest neurological soft signs 20

Another type of the indirect method involves the analysis of cognitive tests An example is the

Wechsler Intelligence Scale for Children (Revised) (WISC-R) WISC-R which compares ldquoverbalrdquo

versus ldquoperformancerdquo 20

1193 Direct Method

The third method which seems most reliable is the direct method which involves the use of

characteristic decoding and spelling patterns to determine the specific dyslexia It has the

additional advantage of addressing the fact that dyslexia is heterogeneous 20

19

120 Functional Imaging Techniques used in the Diagnosis and Study of

Dyslexia

There are functional imaging techniques for diagnosing and studying brain activity in dyslexia

These techniques include positron emission tomography (PET) Magnetic resonance imaging

(MRI) and computed tomography (CT) The two major techniques include

1201 Positron Emission Tomography (PET)

PET is a non-invasive imaging technique that uses radioactively labeled compounds to

quantitatively measure metabolic biochemical and functional activity in living tissue It

assesses changes in the function circulation and metabolism of body organs 54 55 The PET is

a sophisticated nuclear medicine technique that looks directly at cerebral (brain) blood flow

and indirectly at brain activity (or metabolism) The most commonly used labels are

flourophores which emit light when stimulated with light of the appropriate wavelength and

radio nuclides which emit gamma rays or beta particles when they decay56 The dose of

radionuclide injected is minute and does not pose any significant hazard Various isotopes are

used depending upon the brain region and function studied54

Unlike MRI or CT imaging which primarily provide images of organ anatomy PET measures

chemical changes that occur before visible signs of disease are present on CT and MRI images 54 While PET is most commonly used in the fields of neurology oncology and cardiology

applications in other fields are currently being studied 57

1202 Functional Magnetic Resonance Imaging (fMRI)

Functional magnetic resonance imaging (fMRI) is a noninvasive usually painless procedure that

uses a powerful magnetic field radio waves and a computer to produce detailed pictures of

organs soft tissues bone and virtually all other internal body structures The images can then be

examined on a computer monitor or printed The fMRI does not use ionizing radiation57 58

Functional magnetic resonance imaging (fMRI) is a relatively new procedure that uses MR

imaging to provide a picture of the brains active rather than its static structure The fMRI

20

machines measure the level of oxygen in the blood through a technique called blood level-

dependent (BOLD) contrast The two different states of hemoglobin-oxygen-rich oxyhemoglobin

and oxygen-poor deoxyhemoglobin differ in their magnetic properties The huge fMRI magnets

are sensitive to changes in the concentration of deoxyhemoglobin As neural activity increases

blood flow to the vasculature of the brain presumably increases altering this concentration The

more active a brain area is the more blood flows to it Thus fMRI can provide a moment-by-

moment movie of brain activity

Studies using fMRI are beginning to shed light on dyslexia Functional magnetic resonance

imaging (fMRI) is similar to MRI as both procedures use a similar scanner The difference is that

the fMRI uses somewhat more sophisticated hardware and software that allow it to capture brain

changes (mainly blood flow) as a person performs a specific cognitive task The functional

magnetic resonance imaging (fMRI) scanner measures where blood flows through the brain

during certain tasks26

Earlier studies of dyslexia were based on histo-pathological studies According to Habib37 the

fMRI has an added advantage that cortical anatomy of the dyslexic brain can be demonstrated

thus answering the criticism addressed to results from pathological findings

120 Classifications of Dyslexia

Dyslexia can be classified into two broad categories according to its presumed aetiology The

following methods of classification have been documented by Helveston59

1 Primary dyslexia (specific developmental dyslexia)

2 Secondary dyslexia

1211 Primary Dyslexia

This is considered to be caused by a specific central nervous system defect This neural defect is

thought to be located in the angular gyrus of the dominant hemisphere It is often hereditary and

21

may affect more boys than girls In primary dyslexia the decoding process involved in language

system is affected and reading becomes extremely difficult for the child However intellectual

tasks that do not involve reading text are unaffected 59

1212 Secondary Dyslexia

There are two types of secondary dyslexia59 Endogenous and Exogenous

a Endogenous Dyslexia this results from pathological changes in the central nervous

system secondary to trauma or disease such as childhood meningitis hydrocephalus

with brain damage and porencephalus59 Other medical causes include prematurity

congenital hydrocephalus encephalitis traumatic brain injury and lead or

methylmercury poisoning50

b Exogenous Dyslexia this results from intellectual deprivation usually limited to lack of

adequate educational opportunity or motivation59 or due to low intelligence socio-

cultural deprivation primary emotional problems sensory impairment (visual or

auditory) poor motivation or attention problems20 Reading disability due to these

mentioned factors have been referred to as non-specific reading disability20

1 22 Sub-types of Dyslexia

When children first start to read they learn to recognise simple words by their shapes (sight

analysis) building their sight vocabulary This is followed by learning how to analyse

complicated words by breaking them down into their sound components (phonetic analysis)

The dynamics of the two processes (Sight-analysis and Phonetic analysis) are important in

explaining how words are decoded based on the neurologic-behavioural model documented by

Christenson et al 20 and Evans 21 Dyslexia can be divided into the following subtypes 20 21

22

1221 Dysphonetic (auditoryphonetic)

The individual has a minimal dysfunction involving the Wernickes area Readers have an

extremely difficult time developing phonic word analysis or decoding skills Their typical

reading errors are wild guesses (lsquolikersquo for lsquolittlersquo) errors picking words with similar shapes

(horse for house ) and semantic substitutions (home for house) Their typical spelling errors

are phonetically inaccurate (reffor rough) This type of reader persists in using whole-word

approach and will guess at unfamiliar words rather than using their word analysis skills While

they do not often continue to reverse letters and words after the age of eight they often make

spelling reversals (For example interver for inventor and wirters for writers) Typically

they often look at the first and last letter and the length of the word and then guess (monkey for

money or stop for step)20 21

1222 Dyseidetic (visual-spatial)

In dyseidetic type of dyslexia dysfunction occurs in the angular gyrus The individual have high

ability in phonic word analysis They often have a difficult time learning the letters of the

alphabet They usually have a very low sight vocabulary poor visual memory poor perceptual

skills and letter and word reversals (dig for big or saw for was) Their reversals often last

past the age of eight and include both reading and writing reversals This type of child is often a

very slow reader who often will try to sound out familiar as well as unfamiliar words They spell

the word as it sounds By first identifying the learning style of these children probably during the

case history the optometrist may know the childs strengths and weaknesses Vision therapy can

then be designed to fit each childs needs Enhancing the childrsquos visual perceptual and eye

tracking skills plus having the schools teach these children according to their strengths enable

them to achieve their full potential in school 20 21

122 3 Dysnemkinesia (motor)

Dysnemkinesia is a less serious form of dyslexia and it involves minimal dysfunction of the area

of the motor cortex involved in letter formation Dysnemkinesics can be distinguished by their

23

frequent letter reversals such as d for b as in doy for boy Additional dyslexic types occur

when combinations of the three basic types arise such as dysphoneidesia dysnemkineidesia

dysnemkinphonsia and dysnemkinphoneidesia20 21

1224 Dysphonetic-Dyseidetic

Dysphonetic- dyseidetic reader has a weakness in both whole-word and phonic analysis Their

reading errors include wild guesses (fish for father) and word reversals (no for on) Their

spelling is usually phonetically inaccurate and often bizarre (rit for faster) 2021

123 Theories of Dyslexia

Assisted by the increasing body of knowledge obtained by neurophysiological and imaging

studies14 several theories have been proposed with the aim of characterizing the fundamental

processes underlying dyslexia14 These theories can be classified under four major frameworks

phonological visual cerebellar and auditory

1231 PhonologicalndashDeficit Theory

According to the phonological theory affected individuals have difficulties in perceiving and

segmenting phenomes leading to difficulties in establishing a connection between phonemes and

graphemes 14 The brain recognizes language in a hierarchical order The upper levels deal with

semantics (the meaning of words) syntax (grammatical structure) and discourse (connected

sentences) The lower levels of hierarchy deal with breaking sounds into separate small units

called phonemes (sound units) Thus before words can be comprehended at higher levels in the

hierarchy it has to be broken down into phonologic constituents that the alphabetic characters

represents15 26 An adequate reading development stems from some considerable level of spoken

language already acquired by a child in the early years of life Consequently failure to develop

the association between letters (grapheme) and sound (phoneme) has been considered to be a

major cause of reading and spelling impairment in most cases of developmental dyslexia60-62

24

Reading requires some skills of phonological processing to convert a written image into the

sounds of spoken language1 According to the phonologic-deficit hypothesis people with

dyslexia have difficulty developing an awareness that words both written and spoken can be

broken down into smaller units of sound and that in fact the letters constituting the printed word

represent the sounds heard in the spoken word2

According to Habib37 neuropsychological studies have provided considerable evidence

indicating that the main mechanism leading to reading difficulties is phonological in nature and

that it has been repeatedly demonstrated that the core deficit responsible for impaired learning to

read is phonological in nature and has to do with oral language rather than visual perception At

the brain level this cognitive deficit would arise from a congenital dysfunction of certain cortical

areas involved in phonology and reading60 At the neurological level it is usually assumed that

the origin of the disorder is a congenital dysfunction of the left-hemisphere perisylvian brain

areas underlying phonological representations or connecting between phonological and

orthographic representations60

Brain imaging studies in dyslexics in response to a phonological task indicate under-activation of

posterior brain regions (Wernickesrsquos area angular gyrus and striate cortex) and relative over-

activation in anterior regions (inferior frontal gyrus) These brain activation patterns provide

evidence of an imperfectly functioning brain system for segmenting words into their phonologic

constituents39 42 Since most dyslexics show deficits in phoneme processing the phonological

deficit is the most significant consistent marker of dyslexia and remains the predominant theory15343739 hence the focus on phonology in the majority of remedial programmes 63

Based on the phonological deficit theory the other symptoms of dyslexia are considered as

simple co-morbid markers that do not have a causal relationship with reading disability62

However proponents of the phonological theory typically contend that these disorders are not

part of the core features of dyslexia62 A major setback with the phonological theory is that it

does not account for symptoms such as impairment in visual perception and problems with motor

coordination that are found in dyslexics 15 and does not account for symptoms of dyslexia that are

unrelated to phonetic decoding difficulties such as problems with short term memory and visual

25

processing issues64

The two basic processes involved in reading are decoding and comprehension In dyslexia a

deficit at the level of the phonologic units impairs the ability to segment the written word into its

underlying phonologic elements Consequently the reader experiences difficulty initially in

decoding the word and then in identifying it The phonologic deficit is independent of other non-

phonologic abilities especially the higher-order cognitive and linguistic functions involved in

comprehension such as general intelligence and reasoning vocabulary and syntax are generally

intact A deficit in phonologic analysis and an ironical intact higher-order cognitive ability offers

an explanation for the paradox of otherwise intelligent people who experience great difficulty in

reading 26

According to the phonological model a deficit in a lower-order linguistic (phonologic) function

prevents access to higher-order processes and to the ability to draw meaning from text The

problem is that the person cannot use his or her higher- order linguistic skills to access the

meaning until the printed word has first been decoded and identified Suppose for example that

a man knows the precise meaning of the spoken word apparition however until he can decode

and identify the printed word on the page he will not be able to use his knowledge of the

meaningof the word and it will appear that he does not know the words meaning 26

1232 Cerebellar Theory

The normal pattern of cerebellar asymmetry is anomalous in dyslexia 13 The cerebellar deficit

theory suggests that the automatisation of cognitive processes and motor control in the

cerebellum are disturbed in individuals with dyslexia14 15 62 Evidence for this theory comes

from poor dyslexic performance in coordination balance and time estimation The cerebellar

theory is related to cerebral dominance (greater proficiency of each cerebral hemisphere in

acquisition and performance of certain specific functions) and lateralization (one-sided location

of brain function)13

Physiologically the brain operates cross-laterally thus the left part of the brain controls the right

side of the body and vice versa The human brain is normally asymmetrical due to the

26

specialized functions in the left and right brain However the dyslexic brain appears to be more

symmetrical than the normal brain The consequence of this is that both sides of the brain may

be competing to handle language and therefore less efficient in handling processing language

The left hemisphere is especially important for the production of language The right hemisphere

also has some language capabilities but has no ability to analyse speech sound Secondly the

cerebellum plays a role in motor control and therefore in speech articulation It is postulated that

retarded or dysfunctional articulation would lead to deficient phonological representations

Furthermore the cerebellum plays a role in the automatization of tasks such as driving typing

and reading A weak capacity to automatize would affect among other things the learning of

grapheme-phoneme correspondences 13-15 62

1233 Visual Deficit Theory

For many years a major view was that reading difficulties are caused by dysfunction in the

magnocellular subsystem 65-67 The visual theory does not exclude the phonological deficit but

emphasizes a visual contribution to reading problems in some dyslexics The biological basis of

the proposed visual theory is derived from the division of the visual system into two sub-systems

which carry information to the visual cortex the transient and the sustained systems62

Anatomically the magnocellular pathway receives input from both rods and cones across the

retina and extends from ganglion cells in the retina to the two ventral layers of the lateral

geniculate nucleus (LGN) The large cell bodies in these two layers give the pathway its name It

projects via the visual cortex and the dorsal or ldquowhererdquo stream to the parietal cortex The

parvocellular (or sustained) system on the other receives input from cones and therefore mainly

the central retina and runs from retinal ganglion cells to the smaller cell bodies of the four dorsal

layers of the LGN It projects via the visual cortex and the ventral or ldquowhatrdquo stream to the

infero-temporal cortex and mediates colour vision and detection of fine spatial details61 The

magnocellular system is the first to respond to a stimulus and relies on further detail from the

parvocellular system (Table 12)

27

Table 12 Features of the Magnocellular and Pavocellular Systems2168

Magnocellular(transient) System Pavocellular (sustained) SystemLarge cells Small cells

Dim light Bright light

High contrast sensitivity Low contrast sensitivity

Low spatial frequency High spatial frequency

Peripheral Central

Right brain Left brain

Midbrain Cortical

Non-cognitive Cognitive

Emotional Logical

Hyperopia Myopia

Gross depth perception Stereopsis

Achromotopsia Colour vision

Fast transmission Low transmission

Sensitive to large stimuli Sensitive to small stimuli

The transient system is vital to the timing of visual stimuli and plays an important part in

detecting the movement of objects (motion perception) and ultimately their signals are used to

control eye and limb movements made in relation to visual targets1521 Impaired function of

magnocellular pathway will lead to destabilization of binocular fixation which results in visual

confusion and letters then appear to move around

Dyslexicsrsquo eyes are unsteady when they are attempting to view small letters hence their vision is

unstable they tend to make visual reading errors15 There is a tendency to superimpose letters on

top of each other and mis-sequence letters in a word1621 Evidence for magnocellular

dysfunction comes from anatomical studies of dyslexic individuals showing abnormalities of the

lateral geniculate nucleus1562 psychophysical and neuroimaging studies 62 64 showing decreased

sensitivities in the magnocellular range (low spatial frequencies and high temporal frequencies)

in dyslexics

28

A deficit in the magnocellular system slows down the visual processing system resulting in an

unstable binocular control inaccurate judgements of visual direction a tendency to superimpose

letters and mis-sequence of letters in a word16 21 Some studies65-67 have shown evidence in

support of the magnocellular deficit theory of dyslexia

1234 Auditory Processing Theory

The auditory processing theory specifies that the deficit lies in the perception of short sounds and

fast transitions (called the ldquorapidrdquo or ldquotemporalrdquo auditory processing deficit) Such a

characterization of the auditory dysfunction is consistent with the magnocellular theory since

magnocellular cells are particularly sensitive to high temporal frequencies60 62

Proponents of the auditory processing theory hypothesized that impaired perception of brief

sounds and transitions would be particularly detrimental to speech perception hence would

undermine the development of the childrsquos phonological representations Contrary to this

hypothesis Rasmus60 reported that recent studies have now adequately confirmed that there is

no reliable relationship between performance on rapid auditory processing tasks and speech

categorization and discrimination neither is there a reliable relationship between any auditory

measure (speech or non-speech) and more general measures of phonological skill or reading

ability even when assessed longitudinally A possibility is that the most auditorily impaired

dyslexics also have severely impaired phonology and reading although the reverse is not

necessarily true 60

Despite the fact that evidence exist in support of the theories and that some of the deficits found

in affected individuals are correlated with reading and spelling they may not be causally

associated with dyslexia However results from genetic research may have the potential to help

delineate which cognitive and neurophysiological processes are causally related14

124 Cultural Differences in Dyslexia

The recognition of dyslexia as a neuro-developmental abnormality has been influenced by the

belief that it is not a specific diagnostic entity because it has variable and culture-specific

29

manifestations 69-71 Recent neuro-imaging studies39-42 have demonstrated that impaired reading

of alphabetic scripts is associated with dysfunction of left temporo-parietal brain regions

However it has been reported by Siok et al in two separate studies69 70 that in contrast to the

assumption that dyslexia in different languages has a universal biological origin their study using

functional magnetic resonance imaging with reading-impaired Chinese children and associated

controls showed that functional disruption of the left middle frontal gyrus is associated with

impaired reading of the Chinese language (a logographic rather than alphabetic writing system)

The neural connections involved in reading and reading disorders may vary in different languages

because of differences in how a writing system links print to spoken language69 70 In Chinese

for example graphic forms (characters) are mapped to syllables which differ markedly from an

alphabetic system (such as English) in which graphic units (letters) are mapped to phonemes

These differences can lead to differences in how reading is supported in the brain69 70 Readers

of Chinese show relatively more engagement of visuo-spatial areas and left middle frontal

regions for verbal working memory presumably for recognizing complex square-shaped

characters whose pronunciations must be memorized instead of being learned by using letter-to-

sound conversion rules69 70

In two separate studies Siok Perfetti and Tan69 and Siok Zhen Charles Perfetti and Tan 70

studied language-related activation of cortical region of Chinese dyslexic school children using

the functional magnetic resonance imaging and found a reduced activation in the same left

middle frontal gyrus region in Chinese dyslexics Chinese dyslexics by contrast did not show

functional or structural differences from normal subjects in the more posterior brain systems that

have been shown to be abnormal in alphabetic-language dyslexics69-70 The authors concluded

that the results of the studies suggested that the structural and functional basis for dyslexia varies

between alphabetic and non-alphabetic languages and that the findings provided an insight into

the fundamental patho-physiology of dyslexia by suggesting that rather than having a universal

origin the biological abnormality of impaired reading is dependent on culture

The study by Paulesu Demonet Fazio and McCrory71 gave another perspective Using the

positron emission tomography to study brain activity of dyslexics from three countries (Italian

French and English) the authors found the same reduced activity in a region of the left

30

hemisphere in dyslexics from all three countries with the maximum peak in the middle temporal

gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus

Their study showed that there was a universal neuro-cognitive basis for dyslexia and that

differences in reading performance among dyslexics of different countries are due to different

orthographies Languages with transparent or shallow orthography (such as Italian) the letters

of the alphabet alone or in combination are in most instances uniquely mapped to each of the

speech sounds occurring in the language Learning to read in such languages is easier than in

languages with deep orthography (such as English and French) where the mapping between

letters speech sounds and whole-word sounds is often highly ambiguous 71

125 TreatmentIntervention

Individualized instruction of dyslexics emphasizing increased phonologic awareness can have a

favorable long-term effect on academic achievement Individualized instruction aimed at

increasing phonologic awareness decoding skills sight word vocabulary and reading

comprehension are particularly helpful to school aged children experiencing difficulties with

reading Most importantly intervention (provided by expert teachers) should be initiated early50

Reading disabilities require lifelong assistance and optimal management strategies differ

depending on the patientrsquos age and circumstances In early childhood the focus is on

remediation of reading often with an emphasis on increasing phonologic awareness Other

strategies include using audio books and modified homework assignments For secondary and

college students intervention focuses on accommodations which include extra time for reading

tape recorders in the classroom audio books instruction in word processing and the use of a

spell-checker (poor phonemic association also causes problems in spelling)3450

126 Role of Optometry in the Multidisciplinary Management of

Dyslexia

The role of the optometrist in the multidisciplinary management of dyslexia starts with an attempt

31

to identify characteristics which may indicate that the child may be dyslexic A complete visual

examination should be conducted Any vision anomalies detected should be corrected and a

prompt referral should be made to the appropriate specialist in the diagnosis of dyslexia and a

follow up should be made by the optometrist

A comprehensive visual evaluation should include1 72

1 Evaluation for a sensory problem or refractive anomaly

2 Evaluation for binocular vision problems

3 Evaluation for ocular pathology

4 An evaluation of visual motor skills (required for drawing and handwriting) eye movement

control skills and visual imagery skills

5 Evaluation of visual information processing including visual spatial skills

(rightleft discrimination) visual analysis skills (matching and discrimination skills)

The role of the optometrist in the management of reading dysfunctions is to identify existing

vision disorders which can be achieved by evaluating the following aspect of vision functions

1261 Case History

A detailed case history is paramount for children who present with reading dysfunction A

thorough investigation of the pre-natal peri-natal and post-natal risk factors is important to

establish the possibility of major developmental delay For example maternal history of

alcoholism may indicate foetal alcohol syndrome Prematurity (early birth date of less than 37

weeks or low birth weight of less than 2500g) is also a factor in later learning A working

knowledge that may not necessarily mean expertise in all related areas may be adequate for the

optometrist72

1262 Refractive and Binocular Functions

A thorough investigation and compensation for refractive and binocular vision functions should

be undertaken4472

1263 Visual-Motor Skills

32

VisualndashMotor integration is the ability to coordinate visual information-processing skills with

motor skills44 It is a very vital skill in academic achievement4472 Children suffering from visual-

motor dysfunction have difficulty copying written work accurately and efficiently Difficulty

with copying can reduce the speed and accuracy of writing which subsequently affects

learning4472 The Beery Test for Visual-motor Integration is particularly useful for the primary

care optometrist It requires only a few minutes to administer with an added advantage that the

scoring guide provides many examples of the pass-fail criterion for each item tested72

1264 Visual-Spatial Analysis

ldquoThe visual-spatial skills allow the child to make judgments about location of objects in visual

space in reference to other objects and to the individuals own body It develops from awareness

within the individualrsquos body of concepts such as left and right up and down and front and back It is important for good motor coordination balance and directional sensesrdquo 72 The Rosnerrsquos

Visual Analysis Skills programme is useful for the assessment of both visual-motor skill and

spatial analysis72

1265 Short- Term Visual Memory (Visual-Motor recall)

The visual memory test assesses the ability to recognize and recall visually presented material 4473 The ability to copy written material accurately is more or less a matching process when the

written materials are close (such as copying from workbook to paper) When the reading

material is separated from the actual visual-motor task such as copying from chalkboard to

paper the process becomes more complex An example of short-term visual memory skill

assessment test is the Getman-Henderson-Marcus Test of Visual recall72

1266 Auditory Analysis Skills

Screening for auditory-perceptual skill at the primary care level is essential as the ability to

analyze spoken language into separate sounds and sound sequence is directly related to reading

and spelling success Examples of tests to measure auditory perceptual skills are the Test of

33

Auditory Analysis Skills (TAAS) (which is easy to administer and score) and a more complex

one Test for Auditory-Perceptual Skills72

127 Optometric Treatment Options

The optometric treatment options for the correction of vision defects include the following

options

1271 Correction of Vision Defects

1 Compensation for refractive error

2 Treatment of visual efficiency deficits with lenses prisms and vision therapy

3 Treament of vision information processing deficits with vision therapy commencing with

visual spatial orientation then visual analysis and concluding with visual-motor

integration

4 Referral to another health care professional educational system and or psychologist

should be considered at any time if underlying physical or neurological problems cognitive

deficits or emotional disorders are suspected1 50 72

The expected outcome of an optometric intervention is an improvement in visual function with

the reduction of vision anomalies associated with reading which makes educational

intervention easy1 Solan47 proposed that the role of optometrists should not end with

correcting vision anomalies in dyslexic patients but that optometrists should participate in

community services to assist with the developmental environment of those whose genetic

make-up may predispose them to having reading difficulties

1272 The Use of Tinted Lenses and Coloured Overlays

The patients rely on professionals to attend to their vision care needs Consequently it is

imperative that we practice at the highest level of professionalism and ensure that anything we

prescribe for our patients are supported with credible research

As reported by Evans74 75 Olive Meares in 1980 suggested that some childrenrsquos perception of

text and certain cases of reading difficulties were influenced by print characteristics which

34

subsequently resulted in visual perceptual anomalies such as words blurring doubling and

jumping Subsequently Meares and Irlen proposed that ldquoScotopic sensitivity syndrome or

(Meares -Irlen syndrome) ndash a syndrome of visual symptoms and distortion can be alleviated

with coloured filters Meares and Irlen claimed that the coloured overlays improve reading

ability and visual perception increase sustained reading time and eliminate symptoms

associated with reading such as light sensitivity eyestrain headaches blurring of print loss of

place and watery eyes75 This type of symptoms is reported both by people who experience

frequent severe headaches of the migraine type as well as some people with dyslexia76

Following this initiative Irlen developed a proprietary system in countries like the United States

of America United Kingdom and Australia to detect and treat Meares-Irlen syndrome Meares-

Irlen syndrome is reported to be treated with coloured filters either coloured sheets (overlays)

placed on the page or colored lenses Computer users may attain a similar effect by changing the

colour of the screen background and or text while people that consistently work under the same

lighting conditions can benefit from the colour therapy by simply varying the colour of the

illuminating light75

The earlier studies of Irlen was criticized for its lack of published double masked studies with a

placebo control to support her claims that the colour that helped had to be specific for each

person76

According to Lightstone76 the Intuitive Colorimeter (developed by Professor Wilkins) of the

Medical Research Council in the United Kingdom was the first to be used to conduct a masked

trial With this instrument it was possible to determine the optimum colour for the spectacle lens

without placing lenses in front of the eyes and could be done in a way that the subject is unaware

of the exact colour they had selected At the end of the study it was felt that the use of colour

was credible and was then included as part of a routine assessment of people with specific

learning difficulties at the Institute of Optometry London76

As described by Lightstone76 the Intuitive colorimeter is basically a large box which the patient

or subject looks into The space that the subject looks at is entirely diffused by a coloured light

and so creates the effect of wearing coloured lenses There are no other colours in the field of

35

view and so no reference colours are available to help the subject decide the exact hue (colour) of

the light that they are viewing This encourages rapid colour adaptation The person operating

the colorimeter changes all aspects of the colour that the subject is viewing and the hue

saturation (depth of colour) and the brightness of the colour can be varied independently of each

other Although the procedure is subjective the technique shows whether there is a specific

colour that gives relief from the symptoms If the effect is placebo the results tend to be variable

and no specific colour can be selected as the optimum colour75 76

An advantage of the Intuitive colorimeter is that because of colour adaptation the person is

unaware of the exact color of the light shining on the text74 However a major demerit with

double-masked trial was that the results were purely subjective and there were no objective

measurement of any improvement with the colour 76

128 Potential Mechanism for Benefit from Tinted Lenses

Some theories have been proposed to be the mechanism of benefit from tinted lenses However

Evans 49 suggested that these hypotheses have not been able to account for the high degree of

specificity of the required colour which has been emphasized by Irlen and substantiated by

double-masked randomized placebo-controlled trials

The theories are

a Pattern Glare The basis of the theory on how pattern glare explains the benefits of tinted

lenses is that certain cells within the visual cortex are hypersensitive and so causes

discomfort while viewing patterns of repeating black and white lines on a page 75 Since

some of the neurons in the visual cortex are sensitive to specific colours varying the colour

of the illuminating light may change the pattern of excitation within the cortex This could

account for the benefit from specific coloured filters From this it has been hypothesized

that by changing the colour of the input the stimulus is transmitted to other cells that are

not hypersensitive and so prevent the distortions and discomfort occurring 7576

b Syntonics Syntonics is a colour vision therapy system used by some optometrist for the

treatment of several conditions including strabismus amblyopia and reading disorders

36

The therapy involves subjects viewing a light source covered by a colored filter The theory

is that the colour influences the endocrine system through the inferior accessory optic tract

The colour seems to be chosen on the basis of a hypothetical connection between an alleged

autonomic nervous system imbalance and the type of heterophoria Treatment is said to

result in improved visual fields7475

c Placebo Effect The placebo effect is one of the possible mechanisms for the benefit from

tinted lenses People may believe that a treatment works simply because it is claimed to be

effective74

Williams and Kitchner77 conducted a literature review on the use of tinted lenses and colored

overlays for the treatment of dyslexia and other related reading and learning disorders and

concluded that

1 there was evidence that the symptoms associated with Irlen syndrome are related to

identifiable vision problems such as binocular and oculomotor problems and that condition

returned to normal when treated with lenses prisms or vision therapy When patients

exhibiting the Irlen syndrome were treated with vision therapy the symptoms were relieved

and therefore did not demonstrate the need for colored lenses

2 most investigators did not control for the presence of vision anomalies

3 the results of prospective controlled research on the effectiveness of tinted lenses or colored

overlays varied

4 the results of testing used to determine the most appropriate color were not repeatable

5 the effects of spectral filters and colored overlays were not solely a placebo Colored

overlays and tinted lenses are not cures for dyslexia but may be useful reading aids for

some individuals with reading difficulty

6 there was a lack of agreement about the best ways to evaluate patients for the presence of

Irlen syndrome

The American Optometric Association78 issued a policy statement on the use of tinted lenses and

colored overlays which states as follows

37

1 Undetected vision problems may be a factor in individuals who exhibit the symptoms of

the Irlen Syndrome A comprehensive eyevision examination with particular emphasis

on accommodation binocular vision and ocular motor function is recommended for all

individuals experiencing reading or learning difficulties as well as those showing signs

and symptoms of visual efficiency problems

2 The American Optometric Association encourages further research to investigate the

effect that specifically tinted lenses and colored overlays have on visual function related

to reading performance

3 Vision problems are a frequent factor in reading difficulties Ignoring the role of vision

or inadequately evaluating the vision of individuals with reading problems is a disservice

which may prevent the person from receiving appropriate care

In concluding Lighthouse76 advices that until more is known about prescribing specific colored

lenses optometrists should be certain to offer the patient the best advice According to Wilkins 79

some 20 of unselected children in mainstream education (not simply those with reading

difficulties) find colored overlays of benefit and the benefit is pronounced in 5 Wilkins79

emphasized that ldquocolored filters do not provide a treatment for dyslexia ndash they provide a means of

avoiding the visual stress with which reading is sometimes associatedrdquo

129 SUMMARY

Dyslexia is a neuro-developomental condition that impacts on a persons spelling

comprehension writing and reading and not only limits their academic career but also impacts

on all areas of life that require these skills Its diagnosis management and treatment require a

multidisciplinary team as it can present with many characteristics which requires careful

classification While it is not always possible to identify the cause it is generally considered to

be a neurological condition with a genetic origin It affects up to one fifth of Caucasians with

limited studies being done on other ethnic groups including Africans

The aim of the study was therefore to investigate the prevalence of vision defects in an African

38

South African population of dyslexic schoolchildren with an attempt to identify high risk

visual conditions that may be prevalent It was hypothesized that there was no relationship

between vision defects and dyslexia

The reading process requires learning to transform printed letters on a page to words that have

meaning Words are made up of phenomes which are represented by letters and it is the

inability to break words into parts that result in the manifestation of dyslexia Its characteristics

include letter reversal faulty pronunciation difficulty with word retrieval and learning to talk

As all visual stimuli is transmitted through the eye there is an association between ocular

conditions and problems associated with letter words and reading hence the involvement of

optometrists The primary role of optometry is to identify and compensate for visual defects

that may constitute an impediment to reading Evaluation by an optometrist is therefore

important to assess and provide advice or devices where appropriate

Initial identification usually takes place by teachers or parents and may result in the child

being sent to a school equipped to deal with learning disabilities Psychologists diagnose

dyslexia using some range of psychometric tests The advent of imaging techniques such as

positron emission tomography and functional magnetic resonance has aided research in

dyslexia as these techniques enable the study of dyslexic brain activity as a reading task is

performed The two broad categories of dyslexia one being due to a genetic neurological

defect and the other being due to a secondary incident that impacts on the brain There are a

number of theories regarding the underlying causes the only theory related (still controversial)

to visual system being the visual processing theory

Chapter One provided the study aims objectives hypotheses to be tested background

information on dyslexia and rationale for the study while Chapter two will review the related

literature on dyslexia and vision

39

CHAPTER TWO LITERATURE REVIEW

21 Introduction

The relationship between vision and dyslexia has been a subject of controversy in optometric

and ophthalmologic literature A review of existing literature 1-32 on the area of vision and

dyslexia reveals that the studies conducted to investigate visual functions in the dyslexic

subjects do not establish a causal relationship between vision and dyslexia but tend to

establish associations between dyslexia and certain vision variables The literature review also

shows that there exists a dichotomy between the ophthalmologists and optometrists on the

subject of dyslexia and vision The difference in perspective can be seen from the style of

report in studies conducted by optometrists 5683-849198100-101 and by ophthalmologists 2-4 85 95

99102 Optometrists maintain that although vision may not be considered as a cause of dyslexia

compensating for visual defects make reading easier for the dyslexic child Ophthalmologists

hold the contrary view that vision does not in any way contribute to dyslexia Dyslexia has

also been thought to be a language-based learning disability80 Consequently from the speech-

language pathologistsrsquo perspective ldquoa child with a speech deficit may also have an emotionally

distorted perspective and a child whose classroom behaviour is hard to control may also be

unable to readrdquo The role of the psychologists is to perform psychometric assessments of the

dyslexic child32 The educator is primarily involved with special educational intervention for

the dyslexic child

Flax 22 proposed that the role of visual factors in dyslexia ldquodepends on the definition of vision

that is consideredPrime meaning that if vision is narrowly defined as visual acuity or clarity of

sight it is unlikely that there would be any relationship between vision and dyslexia If other

peripheral visual functions such as fusion convergence refractive errors and accommodation

are considered there is likely to be a relationship but then questioned the possibility of vision

being a causative factor Flax22 reiterated that the most important aspect of vision that is

closely linked to dyslexia is when vision is defined to include perceptual and integrative

aspects of vision It was concluded that ldquovision difficulties are part of the dyslexic

40

syndrome22

From a similar perspective Solan81 suggested that vision anomalies are contributory and

frequently hinder responses to educational intervention He emphasized that in cases where

vision anomalies are factors involved in the reading disability the effort required to maintain

clear single binocular vision decreases the efficient organization of the visual cognitive

response

This review has been structured in a way that an overview of a study will be made when it is

first cited Subsequently such studies will be cited only with references to the vision variable

being reviewed Only the studies that relate directly to the topic of the present research such as

visual acuity refractive errors near point of convergence ocular alignments accommodation

functions and vergence reserves are reviewed

It is also important to note that there are many variations in the results reported by different

authors In view of this the findings are thus presented on an individual basis Various studies

have attempted to study the relationship between dyslexia and vision and have examined the

different aspects of vision suspected of affecting reading ability However the major

drawbacks that plague the researches in the subject are Firstly intelligence quotient is a factor

to consider when defining dyslexia However only a few studies587-89 consulted have

considered intelligent quotient in their subjectsrsquo selection

Another area worth mentioning is the fact that some of the studies lacked control group which

made the reports difficult to assess and utilize In addition some researchers failed to classify

some visual functions such as binocular coordination and refractive errors thereby making it

difficult to evaluate the impact of a particular vision variable on reading ability Furthermore

some studies failed to indicate the eye examination techniques used in their protocol which

makes the replication of such studies difficultGiven the inherent limitations with studies

conducted to investigate vision in dyslexic children these studies are important in guiding the

teacher and clinicians on the appropriate intervention for the child

22 Visual Acuity

41

Vision plays an important role in the reading process as the ability to see printed material is

crucial in reading Reading efficiency and speed starts to decrease when the print size is less

than three times larger than an individualrsquos visual acuity threshold A marked bilateral

reduction in visual acuity can definitely limit reading performance but it is unknown if

deficient visual acuity characterizes students who have difficulty to read82

Available studies3-58384 that have attempted to see how visual acuity affects reading

performance are presented starting with the first three studies in this review that found a higher

prevalence of reduced visual acuity among dyslexics compared to normal readers

In 1994 Latvala Korhoenen and Penttinen 3 assessed the ophthalmic status of fifty five (55)

dyslexic and fifty (50) control subjects in an age gender and social class matched study in

Finland The subjectsrsquo school grade and intelligence quotient were not indicated Since

intelligence quotient is a major factor in the definition of dyslexia the lack of information on

intelligence quotient may lead to the questioning as to if the population studied were truly

dyslexics The authors assessed visual acuity using the Snellen chart and found that for the

control group all subjects had visual acuity of 07 (69) while in the dyslexic group two

children (36) had bilateral visual acuity of less than or equal to 07

A study to examine the lsquoophthalmologicalrsquo status of school children with dyslexia in Norway

was conducted by Aasved 4 in 1987 Visual examinations were performed on two hundred and

fifty nine dyslexic children The number of the participants from the control group was not

clearly stated The investigation techniques used was also not detailed As a result it may be

impossible for other researchers to replicate the study However Aasved 4 reported that the

dyslexic population had a higher prevalence of reduced visual acuity than the normal readers

did It was concluded that although there was no causal relationship between eye

characteristics and reading difficulties eye abnormalities should be corrected when detected in

dyslexic children

42

Evans et al 5 in 1993 studied some sensory and refractive visual factors in 39 dyslexic and 43

control subjects aged between seven years six months and twelve years three months in the

United Kingdom Participants from both groups had intelligence quotient of above 85 The

authors found that the dyslexic group had a significantly worse near and distance binocular

visual acuity the inter-ocular difference was not statistically different in the two groups and

the difference between the best monocular and the binocular visual acuity was not statistically

different between the two groups at distance or near They claimed that the ldquoreduced visual

acuity may not have been due to refractive defectsPrime Apart from refractive errors a reduced

visual acuity may arise from pathological conditions such as cataract and glaucoma

The following two studies reported similar prevalence of visual acuity worse than 69 In 1980

Hoffman83 investigated the incidence of vision difficulties in learning disabled children in the

United States of America Vision evaluations were performed on one hundred and seven

children with learning problems who were referred to the clinic for vision care from educators

and psychologists The subjectsrsquo mean age was eight years eight months As a comparison

(control) group twenty five patients between the ages of five and fourteen were selected from

the general optometry clinic The prevalence of visual acuity poorer than 69 was reported to

be 1028 among the learning disabled subjects The subjects from the control group were not

appropriately selected not representative enough and were not age-matched although the issue

of the comparison group being inappropriately selected was acknowledged by the author

Another limitation with the study was that no information was given on the eye examination

procedures followed This may be problematic as it makes comparison with other studies and

replication of the study difficult

Sherman84 conducted a study to ldquorelate vision disorders to learning disabilitiesrdquo in the United

States of America in 1973 Thirty-nine boys and eleven girls participated in the study The

children were diagnosed as having learning difficulties by the referring psychologists Their

43

ages ranged from six to thirteen years The technique used to measure visual acuity was not

indicated but it was reported that five children (10) had visual acuity poorer than 69

Other studies reported the prevalence of visual acuity to be similar between the control and

reading disabled group and include the following studies In 1985 Helveston Weber and

Miller85 (from ophthalmology) studied visual functions and academic performance in first

second and third grade children in the United States of America One thousand nine hundred

and ten children participated in this study The Snellen chart and the Jaegar print were used to

assess distance and near visual acuity respectively The authors reported that 94 of the

children tested had normal visual acuity and that visual acuity was not related to academic

performance in a positive way Although the study may be credited with a large sample size a

possible limitation with the application of the findings of this study was that the passfail

criteria were arbitrarily chosen by the examiners ldquoon the basis of previous clinical experiencerdquo

without references to other research studies Secondly the authors failed to present the results

of the statistical analyses of the relationships between vision functions and reading

Grisham and Simon82 conducted a review of literature on refractive error and reading process

and reported that there are abundant and reliable scientific evidence that shows a higher

prevalence of vision defects among reading disabled children than among normal readers It

was emphasized that there are cases where prescription of glasses has improved academic

performance Their analysis revealed no statistically significant difference in distance visual

acuity among normal and disabled readers but that near visual acuity may be an index which

distinguishes some children who are having difficulty to read and those who do not

It is important to note that since reading is a near point task a deficient near visual acuity may

be more contributory to poor reading than reduced distance acuity Consequently Grisham

and Simons82 stated that ldquodeficient distance visual acuity would not be expected to affect the

44

reading skills of children unless substantial reading instructions take place on the chalk board

It is possible that distance acuity is a factor in the early grades when reading is first

introducedrdquo

In 2007 Grisham Powers and Riles86 studied visual skills of 461 poor readers in California

high schools The participants mean age was 154 years and there was no control group in the

study Participants for the study had been identified by the schools as poor readers The vision

functions measured were visual acuity near point of convergence ldquoconvergence and

divergence and fusional rangesrdquo accommodative amplitude and accommodative facility No

information was given on IQ but it was mentioned that the basis for referral of students for

assessment was poor reading performance which was determined by the school and defined as

reading two grade levels or more below grade level Visual acuity was assessed using the

Snellenrsquos chart at six meters The authors reported that 568 of the participants had visual

acuity of 66 in both eyes 261 had visual acuity of 69 in each eye while 172 had visual

acuity of worse than 69 in either eye

Another comprehensive study conducted in 2007 was reported by Kapoula Bucci and

Jurion87 Convergence and divergence functions in 57 dyslexic and 46 non-dyslexic age-

matched children were studied in French dyslexic children The mean age of the dyslexic

subjects was 113 years the mean IQ was 105 and the mean reading age was 891 The mean

age for the control group was 107 years The IQ score of the control subjects was not

indicated The vision functions performed included visual acuity stereo acuity and cover test

near point of convergence and ldquomeasurement of convergence fusional amplitude (divergent or

convergent) Unfortunately the authors did not detail the testing procedures for the vision

functions investigated but reported the results of the study It was reported that all children had

normal visual acuity

In 2008 Alvarez and Puell 88 studied accommodation functions in school children with reading

difficulties in Madrid Spain Eighty seven poor readers (mean age 92 years) and 32 control

(mean age not indicated) participated in the study all in grades three through grade six

Relative accommodation amplitude of accommodation and accommodation facility was

assessed

45

The most current literature reviewed was reported by Bucci Gignac and Kapoula 89 in 2008

The authors studied poor binocular coordination of saccades in dyslexic children in France Of

particular interest to the current review is the assessment of visual acuity and refraction

(though was assessed under cycloplegia) heterophoria near point of convergence and fusional

vergence reserves The study participants comprised of 18 dyslexic children (mean age 114

years and mean IQ was 105) The control group comprised aged-matched non-dyslexic

children mean age 11 2 years There were no details on the method of assessment of visual

acuity and refraction but it was indicated that a cycloplegic refraction was conducted

However the authors reported that all children had normal visual acuity but the results for

refractive errors were not reported

In 1991 a study on stereopsis accommodative and vergence facility was conducted by

Buzzelli 6 in the United States of America Thirteen normal readers of average age thirteen

years three months and thirteen dyslexics average age of thirteen years four months

participated in the study Both groups were matched for gender age and intelligence quotient

The study design was comprehensive The Snellen chart was used to assess the visual acuity

Buzzelli6 found that the dyslexics did not perform worse than the control subjects in visual

acuity

Ygge Lennerstarnd Axelsson and Rydberg2 studied visual functions in a Swedish population

of dyslexic and normally reading children in 1993 Eighty six 9-year- old children matched to

controls with regard to age gender class in school and intelligence participated in the study

VA was assessed with ldquoordinary optotypes chartsrdquo They reported that the subjects from the

control group had a better visual acuity than the dyslexic group at both distance and near and

that the results showed statistically significant differences (distance at p =003) and (near at

p=0005)

A comparative study of dyslexic and normal readers using orthoptic assessment procedures

was conducted by Goulandris McIntyre Snowling Bethel and Lee90 The study was

conducted in 1998 in the United Kingdom and comprised 20 dyslexic and 20 chronological and

46

reading age-matched control participants A unique aspect of the study was that the distance

VA was assessed using the Cambridge Crowding Cards This test according to the authors

was devised to evaluate letter recognition acuity in conditions of lsquocrowdingrsquo when other letters

surround the target letter and that the test is designed to ensure that abnormalities of vision

which would not be revealed reliably using single targets were detected The near acuity was

assessed using the Snellenrsquos chart The use of different VA chart at distance and near may

raise a concern of standardization They reported that the prevalence of distance and near VA

(using a 69 criteria) was similar between the groups

In 1992 Evans Drasdo and Richards91 investigated optometric correlates of reading disability

in 10 children aged between eight and fifteen years who were referred to the optometry clinic

by the child psychologist The authors reported only the statistical values mean binocular

distance visual acuity of + 091 and mean binocular near visual acuity of + 089

47

23 Refractive Errors

In the study by Alvarez and Puell 88 no information was given on the protocol used to assess

refractive error but the refractive error results were reported For the poor readers mean

spherical equivalent refractive errors for the right and left eye were 020 06 and 020 06

respectively Fifty-seven (655) children were emmetropic In the control group mean

spherical equivalent refractive errors for the right and left eye were - 020 08 and - 014

080 respectively Twenty (645) children were emmetropic In contrast Evans et al 91

reported the mean spherical equivalent refractive error (by retinoscopy) of + 077DS for the

(right and left eye was similar)

231 Myopia

Myopia is characterized by a decrease in distance visual acuity Myopia would usually not be

expected to affect reading skills since students can make the necessary adjustments to their

reading positions However from a clinical standpoint unusually high myopia myopic

anisometropia and astigmatism could influence patients near performance and cause some

discomfort to the child There is no evidence in the literature that myopia is associated with

poor reading In the study by Alvarez and Puell88 57 of the children from the dyslexic

group were myopic compared to 194 from the control group

In 1987 Rosner and Rosner 93 conducted a retrospective comparison of visual characteristics of

data for two hundred and sixty one children with learning difficulties in the United States of

Ameria who had been identified by the schools as manifesting learning difficulties (LD) The

control group consisted of four hundred and ninety six children without learning difficulties

The subjects from both groups were of age range between six and twelve years The children

had been examined at the University of Houston Optometry Clinic The data collection

techniques used was not given but it may be assumed to be routine optometric eye examination

48

protocols because the study was conducted at the university optometry clinic The prevalence

of myopia (minus 050 dioptres) was reported to be 54 in the non-learning disabled patients

while 19 of the learning disabled patients were myopic

One limitation with this study however was that all subjects were patients of an eye clinic

where the two main reasons for referral are reduced visual acuity and school learning

problems Consequently the prevalence of visual defects in subjects selected from such a

clinical setting may have been higher than those from a non-clinical setting

In a review study conducted by Grisham and Simon82 the prevalence of myopia was lower in

the reading disabled group than among the normal readers Similarly Grosvenor 94 analysed

data from several studies and concluded that myopia is consistently associated with good

reading performance One limitation with this analysis however that is the conclusion drawn

from the review was based only on studies that found positive relationships between vision

anomalies and reading ability Grosvenor 94 however remarked that ldquomany authors failed to

find the relationship because they used ineffective methods

One of the earliest studies on the subject of vision and reading difficulty was conducted by

Eames 95 in 1948 in the United States of America He studied refractive and binocular vision

anomalies in one thousand (1000)rdquoreading failures one hundred and fifty (150) unselected

children and five hundred ophthalmic patientsrdquo Comparing the poor readers and the

unselected groups (control) he found the incidence of myopia to be same in both groups (4)

Myopia was defined as -100 diopter sphere There was no statistical analysis in that study and

the data collection techniques were vague although it was indicated that no cycloplegia was

used While the study may be criticized as not being current it is the major and early study to

give an insight into the subject of vision and reading disability

Eames95 and Shearer96 reported a similar prevalence of myopia (4) Shearer96 investigated

ldquoeye findings in children with reading difficultiesrdquo Two hundred and twenty two children

participated in the study The subjects were screened using the Keystone telebinoculars The

prevalence of myopia defined as - 050 dioptres sphere was 4 He concluded that vision

49

functions are not related to reading ability

232 Hyperopia

Hyperopia is often associated with poor reading performance probably through the mechanism

of deficient accommodation and poor motor fusion skills 82 A child with a relatively high

degree of farsightedness may not report blur vision at distance due to compensation for

hyperopia through the accommodation mechanism which also is used to create clear vision at

near This results in overuse of the accommodation system both optically and physiologically

and a subsequent fatigue or spasm of the accommodation system 97

One of the studies that classified refractive error in the literature was conducted by Rosner and

Rosner93 They reported that hyperopia appeared to be more prevalent (54) in learning

disabled than in the non-learning disabled children with a prevalence of 16

In the study conducted by Alvarez and Puell88 287 of the dyslexic group was hyperopic

compared to 161 from the control group Grisham and Simon82 in a literature analysis of

refractive errors and reading process reported that the prevalence of hyperopia among poor

readers is higher than among good readers and that the correction of hyperopia resulted in

improved performance

Grosvenor 94 reported that hyperopia is usually associated with poor reading Similarly in

Eamesrsquo study95 43 of poor readers had hyperopia while for the control group only 12 were

hyperopic The author gave no information on statistical analysis Shearer96 reported the

prevalence of hyperopia to be 16

233 Astigmatism

Clinically uncorrected moderate to high astigmatism can cause discomfort and lead to poor

reading performance at near distance Relatively few studies have attempted to study the

relationship between astigmatism and reading disability The prevalence of astigmatism

reported was higher in the dyslexic group compared to the control group in all the studies

reviewed 39395 These studies include the study by Latvala et al 3 who reported that 36 of

50

the dyslexic subjects were astigmatic (of greater than -100 diopter cylinder) and none from the

control group had astigmatism of that magnitude Eames95 reported the prevalence of

hyperopic astigmatism of more than -100 diopter cylinder among the poor readers to be 6

and 4 in the unselected group while the prevalence of myopic astigmatism was the same

(1) in both groups Rosner and Rosner 93 found a higher prevalence (30) of astigmatism

amongst the non-learning disabled subjects compared to the learning disabled group (27) In

Shearerrsquos 96 study of eye findings in children with reading difficulty the total prevalence of

astigmatism classified as myopic astigmatism (3) and hyperopic astigmatism (3) reported

was 6 There was no control group in this study In the study by Alvarez and Puell 88

astigmatism was detected in seven (8) and eight (92) right and left eyes respectively while

astigmatism was detected in five children (161) from the control group Ygge et al 2

reported that the prevalence of astigmatism was higher in the dyslexic group (28 left eye 25

) compared to the control groups (right eye 183 left eye 243) although there was no

statistically significant difference between the groups (p=025)

234 Amblyopia

Only two studies on the available literature consulted reported on the prevalence of amblyopia

both indicating a higher prevalence amongst the dyslexic group compared to the control group

In the study by Latvala et al 3 the prevalence of amblyopia in the dyslexic group was 36 and

none in the control group had amblyopia Although a detailed prevalence of refractive error

was not presented in that study it was reported that nearly all subjects had visual acuity of 07

In relating refractive error to amblyopia the prevalence of astigmatism in the control group

was 36 and none from the dyslexic group had astigmatism Consequently a low prevalence

of amblyopia may be related to fairly good visual acuities in both groups On the contrary

Rosner and Rosner 93 reported that 4 of the non-learning disabled patients had amblyopia

compared to the 3 found in the learning disabled patients Hyperopia is the refractive error

that is more often associated with amblyopia However this study presented a higher

prevalence of amblyopia (4) but a lower prevalence of hyperopia (16) in the non-learning

disabled patients However a difference of only 1 is not high enough to account for the

difference

51

235 Anisometropia

Anisometropia has been thought to be associated with poor reading skills probably through the

mechanism of poor sensory and motor fusion rather than reduced visual acuity It degrades

binocular coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 82

Three studies2395 reported findings on anisometropia Latvala et al 3 reported a higher

prevalence of anisometropia (of greater than or equal to one diopter) of 6 for the control

group than in the dyslexic group with 36 Similarly Ygge et al 2 found a higher prevalence

of anisometropia in the control group (158) than in the dyslexic group (94) Eames 95

reported that 13 of poor readers were anisometropic while 6 from the control group had

anisometropia According to Grisham and Simons 82 only few studies have considered

anisometropia to affect reading performance The authors concluded that there was insufficient

number of studies to draw firm conclusions from their review

Other studies2598 failed to classify refractive error according to the type These include

studies by Evans et al 5 who investigated some sensory and refractive factors in dyslexia The

distribution of refractive errors was found to be similar in both the dyslexic and control groups

The details of the participants profile was given earlier (section 21) The refractive errors

were assessed by distance retinoscopy and subjective refraction A cross cylinder was used to

measure astigmatism The spherical equivalent refraction was calculated by adding one-half

of the cylindrical components to the spherical component It was not indicated if refraction

was performed under cycloplegia Ygge et al 2 assessed refractive errors objectively using

streak retinoscopy under cycloplegia They reported that the distribution of refractive errors

were similar between the dyslexic and control groups

Sucher and Stewart 98 examined 136 fifth and sixth grade children comprising seventy two

with learning disability and 64 from the mainstream school as the control group The learning

52

disabled children were selected from a classroom of children attending special supervision and

instruction Children from both groups were matched by grade level No information on

intelligence quotient was given The testing protocol used to assess refractive error was not

indicated The authors reported a higher prevalence of refractive error in the dyslexic (208)

than in the control (94) group As indicated earlier refractive errors were not classified

according to the types in the three preceding studies 2598

Hoffman 83 reported the prevalence of refractive error to be 215 There was no report on the

control group and the techniques used to assess refractive errors were not indicated In the

study by Helveston et al 85 only 1 of the participants had refractive error when assessed

objectively with the retinoscope without the use of cycloplegia Refractive errors were not

classified according to the types (myopia hyperopia or astigmatism)

In 1991 Hall and Wick 99 conducted a study to investigate the relationship between ocular

functions and reading achievement in the United States of America One hundred and eleven

children (in grades one through grade six) participated in the study Eleven ocular functions

were investigated including visual acuity saccade eye movements accommodation facility

amplitude of accommodation accommodation posture heterophoria horizontal fixation

disparity vertical fixation disparity near point of convergence titmus and Randot stereopsis

Accommodation facility was measured using the flipper technique The amplitude of

accommodation was determined using the push up to blur technique Heterophoria fixation

disparity and near point of convergence were also measured but the techniques used were not

indicated The age range of the subjects studied was not indicated Using multivariate

correlation they found no statistically significant relationship between the ocular functions and

reading abilities as they noted that the acquisition of reading skills is given by many forces

which include the use of remedial instructions and language skills the role of peer pressure and

innate intelligence

53

24 Near Point of Convergence

Anomalies of convergence is one of the common problems encountered in the optometric

practice especially in the pediatric population and the reports on near point of convergence has

been presented comprehensively in the literatures reviewed

Grisham et al 86 measured the near point of convergence by asking the student if the clown face

target was seen as single when the examiner held it approximately 20 cm in front of the

studentrsquos eyes If so then the student was requested to say when the object appeared to double

as it moved closer to the face The examiner placed one end of an accommodative rule on the

studentrsquos forehead and judged where the two eyes diverged as the target moved toward the

studentrsquos face The measurement was repeated three times The authors found that 846 of

the students had near points at 8 cm or closer which they considered ldquonormalrdquo while 154

had near point of convergence of 9 cm or farther

In the study by Kapoula et al87 the NPC was determined by placing a small pen-light at

3040cm in the mid plane in front of the subject and moving it slowly towards the eyes until

one eye lost fixation The authors reported that the NPC was significantly more remote

(gt10cm) in dyslexics than in non-dyslexics (36 versus 15)

Bucci Gignac and Kapoula 89 followed similar protocol as in the study by conducted Kapoula

et al 87 The near point of convergence was determined by placing a small pen-light at 3040

cm in the mid plane in front of the subject and moving it slowly towards the eyes until one eye

lost fixation It was reported that the NPC was not statistically different between the two

populations the median value was 5 cm for both dyslexic and non-dyslexic children

Latvala et al3 failed to indicate the technique used in assessing the near point of convergence

54

but simply noted that ldquoconvergence ability was measuredrdquo but reported that convergence near

point greater than or equal to 8cm was found in 127 of the dyslexic subjects and 2 of the

control group The authors concluded that the near point of convergence was the only variable

that showed a statistically significant difference between the groups They concluded that

ophthalmic factors should not be overlooked as a contributing factor to dyslexia as they may

constitute part of the dyslexic syndrome and recommended that vision anomalies should be

corrected whenever detected

Shearerrsquos 96 findings are similar to the results reported by Latvala et al 3 He assessed the near

point of convergence using the keystone telebinoculars and reported that 12 of the

participants in his study had convergence problems

In a relatively elaborate study Evans Drasdo and Richards100 investigated accommodative and

binocular functions in dyslexia The subjects were 43 control and 39 dyslexics aged between

seven years six months and twelve years three months An intelligence quotient range of over

85 for the dyslexics and over 90 for the control with the Wechsler Intelligence Scale for

Children (Revised) (WISC-R) was recorded They measured the near point of convergence

using the royal air force (RAF) rule It was concluded that all subjects were able to report a

clear subjective break and recovery and reported no statistically significant difference between

the two groups in either the near point of convergence

Hoffman83 assessed convergence using keystone visual skills tests but grouped the results as

binocular coordination which makes it difficult to analyze and compare while in the study by

Helveston et al 85 98 of the participants had normal near point of convergence

In 2002 Keily Crewther and Crewther 12 investigated the relationship between functional

55

vision and learning to read in Australian school children Two hundred and eighty-four

children (mean age 99 18 years) received a vision screening emphasizing binocular

anomalies associated with discomfort at near (distance and near visual acuity distance vision

ldquochallenged with binocular +1 D lensesrdquo near heterophoria near point of convergence

stereopsis and accommodative facility) Children were classified as normal readers (n = 195)

children with dyslexia (n = 49) or learning disabled children (children who have learning

difficulty with impaired intelligence quotient) (n = 40) It was reported that the near point of

convergence were better in the dyslexic group compared to the normal and the learning

disabled group but found no statistically significant difference between the groups In

constrast Evans et al 91 assessed near point of convergence using the RAF rule and reported a

highly statistically significant relationship between NPC and reading retardation (p=0019)

25 Heterophoria

Heterophoria is a common binocular problem and in clinical practice it is particularly

important when analyzed in conjunction with fusional reserve as it gives an indication of the

zone of comfortable binocular vision

The distance phoria is a clinical measurement of the tendency of the two eyes to deviate from

parallelism when viewing a target usually at six meters When a closer viewing target is

utilized (say at about the individuals reading distance) the resulting near phoria measures the

degree to which the eyes over converges (esophoria) or underconverges (exophoria) to the

plane of the target97 This latent deviation requires fusional vergence to maintain single

binocular vision and at near they involve the accommodation system Smooth functioning of

these systems particularly at near is important for near tasks92

Several studies312858789959698100101 have attempted to investigate the role of heterophorias on

reading ability and all reported remarkably varying findings The assessment of heterophoria

was extensive in the study conducted by Latvala et al 3 Heterophoria at distance was evaluated

56

using the cover test and Maddox rod The prism bars was used to quantify their findings The

Maddox wing was used to assess phoria at near At near they reported a higher prevalence of

esophoria (61) among the control than the dyslexic group (58) a higher prevalence of

exophoria (25) among dyslexics than the control group (122) and a higher prevalence of

vertical phoria (61) among the control than the dyslexic subjects (36) At far distance

they found a higher incidence of esophoria (113) among dyslexics than the control (82)

subjects a higher prevalence of exophoria (2) among the control group than among the

dyslexics (19) and vertical phoria to be more prevalent in the control (41) than in the

dyslexics (38)

Keily et al 12 assessed heterophoria using the Prentice card and reported that there was a slight

but insignificant tendency for the dyslexic subjects to be more exophoric than the normal and

those with learning disability but found no statistically significant difference The test distance

was not indicated

Evans et al 100 in a study of accommodative and binocular functions in 43 control and 39

dyslexics reported that all subjects examined were orthophoric at distance with cover test

except for one dyslexic subject who demonstrated 10 prism diopters right esotropia and two

diopters right hypertropia which changed to 15 prism diopters right esotropia with two prism

dioptre right hypertropia at near At near horizontal phoria results were similar in both groups

Evans Efron and Hodge 101 studied the incidence of lateral phoria in 45 children with specific

learning disability in the United States of America in 1977 The participants were selected from

a psychologistrsquos file The control group comprised of 364 children in second third and fifth

grades attending regular school It was not indicated why there was such a big difference in

the number of participants from both groups The test used in assessing phoria was the

ldquoRocket card which was used in conjunction with red- green spectaclesrdquo They reported a

57

prevalence rate of 1135 in the control group while 222 of the children from the specific

learning difficulty group had heterophoria There was no statistically significant difference

between the groups In concluding it was recommended that phoria conditions may not cause

reading or learning disability but that the presence of such conditions could contribute to visual

perceptual and attention abnormalities

A report by Sucher and Stewart98 contained limited information as the authors did not detail

whether a higher prevalence rate among control (105) than dyslexic (86) was for near or

distance phoria Similarly Eames95 failed to indicate the distance at which the measurement

was made but reported that 33 of poor readers had exophoria and 22 of control were

exophoric but did not indicate whether distance or near

Helveston et al 85 used the Wottring amblyoscope to determine ldquoobjective and subjective angle

of ocular alignmentPrime and reported that 96 of the participants in their study were orthophoric85 The criteria used to classify ocular deviation were not indicated

Another comprehensive report of heterophoria was documented by Shearer96 The subjects

were screened using the Keystone telebinoculars and reported that 1 of the study participants

had vertical phoria (gt1 prism diopter) 2 had esophoria (gt 4 prism diopter) 3 had

exophoria (gt 4 prism diopter) 26 had exophoria at near It was concluded that vision

functions are not related to reading ability Hoffman 83 reported that ninety three (93) subjects

(869) had problems of binocular coordination but failed to indicate whether it was phoria or

tropia

Kapoula et al 87 evaluated ocular alignment using the cover test and reported that at far

distance the median value of phoria was zero for both non-dyslexic and dyslexic children (p =

019) At near the median value of phoria was similar between the two groups (minus2 prism

58

diopters for non-dyslexics and minus15 prism diopters for the dyslexics (p = 019)

Bucci et al89 measured heterophoria at near and far using the cover-uncover test As

documented by the authors exophoria was not statistically different between the two groups (p

= 02) the median was ldquo0rdquo prism diopters for dyslexics and 2 prism diopters for non-dyslexics

No information was given on other aspects of heterophoria

Simons and Grisham92 conducted a literature analysis on binocular vision anomalies and

reading emphasizing studies with positive and negative relationship between binocular vision

and reading The authors concluded that binocular conditions such as esophoria exophoria

restricted ranges of fusion foveal suppression accommodative insufficiency and oculo-motor

disorders at reading range are more prevalent among poor readers than normal readers

In another study Ygge Lennerstarnd Rydberg Wijecoon and Petterson102 investigated

oculomotor functions in a Swedish population of dyslexic and normally reading children in

1993 The study comprised 86 nine -year- old dyslexic children matched to control with regard

to age gender and intelligence Heterophoria was measured using the cover test They

reported that for the dyslexic group ldquo23 pupils exhibited a phoria (264 9 eso- and 14 exo-

phoria)rdquo whereas in the control group ldquo20 pupils had a phoria (232 6 eso- and 14 exo-

phoria)rdquo and that the difference was not statistically significant (p = 034) At near the

incidence of exophoria was 437 for the dyslexic group and 453 in the control group

There was no statistically significant difference (p=055)

26 Strabismus

Several authors34 83-85 90 93102 included the measurement of strabismus in their studies

reporting remarkably varying findings Helveston et al 85 assessed ocular alignment using the

Wottring amblyoscope and reported that only between 2 - 4 of the students had phoria or

59

tropia of more than four prism dioptres Latvala et al 3 reported the prevalence of tropia at near

for the dyslexic group to be 36 and 2 for the control group Hoffman83 assessed ocular

alignment using the keystone visual skills test and reported that 13 subjects (9 exotropes and 4

esotropes) (1214) had strabismus It is difficult to comment if this relatively high

prevalence of strabismus is related to hyperopia as the refractive error prevalence of 215 was

not categorized as to the type In a study by Sherman84 a total of four children (8) were

strabismic classified as three exotropia (alternating) and one esotropia (alternating) Rosner

and Rosner93 in their retrospective analysis reported that in the learning disabled group 51

had esotropia 39 had exotropia while in the non- learning disabled group 8 were

strabismic It was not unexpected to find a higher prevalence of strabismus in this

retrospective study Aasved 4 found that the dyslexics had a higher prevalence of manifest and

latent convergent strabismus than the control subjects

Goulandris et al90 assessed strabismus using the cover test and reported that there was no

statistically significant difference between the two groups in the prevalence of strabismus

Ygge et al 102 reported that at distance manifest strabismus was more frequent among the

dyslexic (8) pupils than the controls (23) but that the difference was not statistically

significant At near the prevalence of manifest strabismus was 8 (all eso-) for the dyslexics

and 35 (all eso-) in the control group

27 Accommodation Functions

271 Amplitude of Accommodation

The accommodation mechanism is extremely important for learning Children who suffer

some anomalies of accommodation are more prone to fatigue quickly and become inattentive

than those who have normal accommodation function According to Flax 22 an ldquoinefficient

accommodation function is a significant contributor to lowered achievement in the upper

grade

60

The findings on various aspects of accommodation functions were diverse Metsing and

Ferreira 103 studied visual deficiencies in children from mainstream and learning disabled

schools in Johannesburg South Africa in 2008 One hundred and twelve (112) children from

two learning disabled schools and eighty (80) children from a mainstream school in Johannes-

burg participated in the study The vision functions examined were visual acuity refractive

status ocular health status accommodative functions (Posture facility and amplitude)

binocularity (cover test vergence facility smooth vergences NPC and ocular motilities (direct

observation) However according to the authors only the results for accommodation

accuracy amplitude of accommodation vergence facility and saccadic accuracy are reported

since only these variables showed statistically significant relationship on the relationship

between the (mainstream and learning disabled) groups Metsing and Ferreira 103 reported

that a high percentage (516 and 533) of low amplitude of accommodation for the right

and left eyes respectively was found in the mainstream group compared to the 291 and

288 in the learning disabled group The relationships between the mainstream group and

reduced amplitudes of accommodation of the right (p=004) and left eyes (p=0001) were

found to be statistically significant (plt005) The relationship between the mainstream group

and reduced amplitude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to

be moderate and that between the mainstream group and the right eyes (Cramerrsquos V= 0286) to

be low

In the study by Grisham et al86 accommodative amplitude was assessed using the

accommodative rule ldquoThe student held an occluder over the left eye The examiner asked if

the student could see the (2030) target at the larger end of the Occlud-A Measure held at about

20 cm from the face If yes then the student was asked whether the target was in focus or ldquonot

blurryrdquo If the student responded that the target was in focus then the target was moved at

approximately 2cm per second toward the studentrsquos face and the student was asked to report

when the target first became blurryrdquo About 638 of the participants had amplitude of

accommodation of 11D which the authors considered equivalent to the expected amplitude of

61

accommodation for the age About 247 had inadequate accommodation amplitude while

115 had ldquoborderlinerdquo amplitude of accommodation

In the study by Evans et al 100 the amplitude of accommodation was measured using the push up

method They reported that the amplitude of accommodation was significantly reduced in the

dyslexic group and that the mean accommodation lag was calculated for each eye but found no

statistically significant difference between the two groups

Alvarez et al 88 assessed monocular accommodative amplitude using the minus lenses method

The subject viewed horizontal line of 2030 letters at 33cm as the examiner introduced minus

lenses in 025D increments until the target first becomes blurred According to the authors the

working distance adjustment used was kept at 250 DS to compensate for minification The

authors reported that monocular accommodative amplitude was significantly lower (p lt 0001)

in the group of poor readers (right eye 91 D 23 left eye 90 D 23) than in the control

group (right eye 105 D 17 left eye 105 D 17)

Helveston et al85 measured the amplitude of accommodation using the push up to blur

technique and reported that ldquoan ability to accommodate more than eleven dioptres was found in

between 90 and 93 of the studentsrdquo Ygge et al 102 assessed amplitude of accommodation

using the RAF rule and reported that the two groups performed similarly A similar technique

was used by Goulandris et al 90 and a lack of statistically significant difference between the

groups was reported

272 Accommodation Facility

In the same study Evans et al 100 reported that the dyslexic group appeared to be slower at a

62

test of accommodative facility The authors discredited the flipper technique as the

unnatural act of altering accommodation without changing vergence They further noted that

the flipper testhellip would still imperfectly reflect the normal visual environment owing to the

absence of a stimulus to vergence and accommodation

Grisham et al86 measured accommodative facility using the 200D lenses flipper technique

and the technique was performed as follows The 2D lenses was flipped each time the

student said the target was ldquoclearrdquo The target for this test was the 2030 target used for

accommodative amplitude The left eye was occluded The student was asked whether the

target was clear when held at about 40 cm If so then the student was told that the test would

be timed and that it should be reported whenever the target was ldquoclearrdquo The examiner then

started a stopwatch and counted the number of times the lenses were flipped in 30 seconds

That number was taken to represent the cycles per minute (cpm) and therefore to represent the

studentrsquos ability to change accommodation over time Based on the criteria used in their study

the authors reported that 76 of the students had adequate monocular accommodative facility

while 236 had ldquoweak accommodative facility of 9 cpm or less and 81 had accommodative

facility of as low as 1 or 0 cpm

In the study conducted by Alvarez et al 88 accommodative facility testing was conducted using

the following procedure The subject wearing his or her habitual correction was seated and

positioned so that the spectacle plane was 40cm from the Bernell Acuity Suppression Slide

(VO9) A 200 D accommodative demand was used with a 2030 test target at 40cm The

patient was asked to view the reduced Snellenrsquos letters 2030 in line 5 and say ldquonowrdquo when the

letters in that line appear clear and single and to report letters missing from lines 4 and 6

during the binocular testing They reported that the binocular accommodative facility values

were significantly lower (p lt 005) in the poor readers group (49 cpm 31) than the control

group (63 cpm 29)

63

Sucher and Stewart 98 assessed binocular accommodative facility at 35 cm using 150 diopter

flippers lens for ldquoone minuterdquo and ldquoused the University of California Berkeley clinic standard

of 20 cycles90s as a ldquocriterion for successrdquo found a higher prevalence of accommodative

infacility in dyslexics (222) than the control group (86)

On the contrary three authors6 12 93 found the control subjects to have better accommodation

facility than the dyslexic subjects The studies are Rosner and Rosner93 found a higher

prevalence of accommodative infacility among the control (10) than the learning disabled

group (9) Similarly Buzzelli6 found that the dyslexic subjects showed better

accommodation facility than the control group To assess accommodation facility ldquopatients

were asked to clear the number 9 on the Bernell Vectogramm Target at 40cm hellip alternately

viewed the target through a + 200DS and a ndash 200 DS lenshellip for 20 cycles) There was no

statistically significant relationship between the two groups

Keily et al12 assessed accommodative facility using +200DS and ndash 200DS flippers lenses

with an N6 fixation target for one minute at 30 cm rather than 40 cm because according to the

authors the children were required to hold the reading material They reported that

accommodative facility was better in normal readers than either of the poor reading groups

In another study Evans and Drasdo104 reviewed ophthalmic factors in dyslexia and commented

that convergence insufficiency accommodative dysfunctions and unstable eye movements are

more common in dyslexic population than in normal readers

Latvala3 simply noted that some aspects of accommodation were difficult to measure and failed

to give any details

64

273 Relative Accommodation

Alvarez and Puell88 measured relative accommodation using a phoropter The subject viewed a

horizontal line of 2030 letters at 40cm with hisher best refractive correction The examiner

introduced minus or plus spherical lenses in 025 D steps binocularly until the patient reported

first sustained blur Negative and positive relative accommodation values were similar in both

groups This was the only study accessed that reported on relative accommodation

274 Accommodation Posture

Evans et al100 assessed accommodation lag using the monocular estimate method of retinosocpy

and reported no statistically significant difference between the dyslexic and the control groups

Metsing and Ferreira103 measured the accuracy of the accommodative response (lag or lead)

objectively using the MEM retinoscopy in normal room illumination and found that the subjects

from the mainstream school demonstrated a high prevalence of lead of accommodation (40

and 405 for the right and left eyes respectively) However in the learning disabled group a

high prevalence of lag of accommodation (98 and 119 for the right and left eyes

respectively) was found The relationships were found to be statistically significant (p lt 005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000)

Sherman84 assessed accommodation amplitude and relative accommodation using minus lenses

and accommodative facility using the flipper techniques The lens powers for the plus and

minus lenses used to assess accommodation functions were not indicated His findings may be

difficult to utilize due to the somewhat unique way of classifying the results as ldquomechanical

problemsrdquo

Flax22 suggested further investigations in the nature of accommodative function in reading

65

disabled children indicating that the use of the push-up method of assessing the amplitude of

accommodation is unreliable due to its lack of sensitivity Flax22 reiterated that

accommodation has a contribution in learning disabilities and that accommodative function

inefficiencies tend to play an increasing role as the youngster moves through school and that

inefficient accommodation function is a significant contributor to lowered achievement in the

upper grade

28 Vergence Facility

Vergences are a clinical measure of an individualrsquos ability to overcome the prismatically

induced optical displacements of a target in space Convergence relates to each eye fixating

closer in space than the real physical location of the target while divergence is the measure of

the eyes to fixate further in space while still maintaining single binocular vision97

Buzzelli 6 assessed vergence facility as follows ldquopatients were asked to align a vertical picture

(box-X-O) slide made of anaglyph material for monocular presentation under binocular

conditions The target measured 21 mm in the vertical meridian and 10mm in the horizontal

meridian The subjects alternately viewed the target through 16 prism base out and 4 prism

base in The test was continued for 20 cyclesrdquo The author reported that the dyslexics

completed the vergence eye movement task significantly slower (300 seconds) than the normal

readers (240 seconds) A statistically significant relationship between the groups was reported

(p lt 005) It was recommended that the possible role of vergence in dyslexia should be

investigated on a larger sample of dyslexic and control groups

Metsing and Ferreira103 assessed vergence facility test at near (40 cm) with the subjects

presented with a vertical row of letters (69) using the 8 PD (BI) and 8 PD (BO) mounted in a

flipper device They reported that the prevalence of poor vergence facility (205) was found

to be higher in the mainstream group compared to the learning disabled group (183) The

relationship between the mainstream group and poor vergence facility (p = 0000) was found

66

to be statistically significant (plt005) A medium to strong relationship was found to exist

between the mainstream group and poor vergence facility (Cramerrsquos V = 0369) It was

concluded that children from mainstream schools are likely to present with poor vergence

facility compared to children from schools of the learning disabled

2 9 Fusional Reserves

The compensatory effect of fusional vergence reserves on phoria is very important as difficulty

with reading is likely to occur when there is uncompensated phoria in conjunction with

reduced fusional vergence reserves 92

A major study was reported by Evans et al 100 which found that negative and positive reserves

were significantly reduced in the dyslexic group relative to the control The ldquoprism vergence

amplitude calculated as the difference between base out and base in break and recovery points

was relatively reduced in the dyslexic grouprdquo They remarked however that several subjects

were unable to appreciate a subjective blur point but the results for the break and recovery

were complete

In the study conducted by Latvala et al 3 the incidence of fusional amplitude greater than or

equal to thirty two prism dioptre at near was higher in the dyslexic (75) than in the control

(61) group At distance they found fusional amplitude greater than or equal to15 prism

dioptre to be higher in the control (122) than in the dyslexic (94) group

Disordered vergence control was studied by Stein Riddell and Fowler 105 in the Uniteed

Kingdom in 1988 The subjects were thirty nine dyslexic children aged eight to eleven years

and a control group of twenty subjects of similar age group The vergence control was

measured using the Synoptophore They reported that two thirds of the dyslexic group had

67

abnormal vergence control According to the authors vergence eye movements never seem to

have been investigated in detail during reading before and that the study was the first in which

eye movement recording have been used to demonstrate differences between dyslexics and

normal readers

According to Grisham et al86 ldquoConvergence and divergence break and recoveryrdquo were

measured at 40 cm with a horizontal prism bar The student was first asked if the clown face

target on the smaller end of a standard optometric measure (Occlud-A-Measure Bernell

Mishawaka Indiana) could be detected If so then the student was requested to say whether

the target appeared single or double Once single vision was obtained the bar was moved

relative to the studentrsquos eye until double images were reported That point was considered

ldquobreakrdquo Then the motion of the bar was reversed until the student reported the image single

and this point was considered ldquorecoveryrdquo This procedure was first performed for base in (BI)

position of the bar over the left eye and repeated with base out (BO) position over the right

eye The authors reported that a large number of students had poor convergence skills 38

break at less than 18 PD and 95 recover at 7 PD or less These values were considered to be

in the ldquoweakrdquo to ldquovery weakrdquo range and remarked that in contrast to convergence lsquomodalrdquo

values for break (14 PD) and recovery (10 PD) were not in the ldquoadequaterdquo range but would be

classified as ldquovery weakrdquo for break and ldquoweakrdquo for recovery As a result very high

percentages of students had poor divergence skills 82 break at less than 20 PD and 60

recover at less than 11 PD

Kapoula et al 87 measured vergence reserves using prism bars at far and near distance for both

groups They reported that the median value for positive fusional vergence was 16 prism

diopters and 14 prism diopters for non-dyslexics and dyslexics respectively at far distance (p

= 012) and 20 prism diopters for both non-dyslexics and dyslexics at near distance For

negative fusional vergence they reported that ldquodivergence amplituderdquo was significantly

different in the two groups for both distances (p lt 0005) At far the median value was 6

68

prism diopters and 4prism diopters in non-dyslexics and dyslexics respectively and 12 prism

diopters and 10 prism diopters respectively at near

Bucci et al 89 measured fusional reserves at far and near using prism bars (base in and base

out) According to the authors orthoptic evaluation of vergence fusion capability showed a

significantly limited divergence capability for dyslexics compared to non-dyslexics the

median value was 10 prism diopters for dyslexics versus 17 prism diopters for non-dyslexics

In contrast convergence amplitude was in the normal range for the two populations while for

dyslexics the amplitude was significantly larger than for non-dyslexics the median value was

30 prism diopters and 18 prism diopters for dyslexic and non-dyslexic children respectively

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups

The mean fusional convergence capacities at distance were 1680 prism diopters for both the

control and the dyslexic group At near the corresponding figures were 2640 prism diopters

and 267 prism diopters respectively The mean fusional divergence capacity at distance was

650 and 620 prism dopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 105 and 102 prism diopters respectively There was no

statistically significant difference between the groups

210 Ocular Pathology

Ocular pathologies do not seem to be prevalent in the dyslexic population and it was not

reported on in the literature reviewed However according to Evans and Drasdo100 the

perspective to relate ocular pathology to dyslexia is in individuals who were normal readers but

developed reading disability due to pathological conditions such as stroke or tumors affecting

the right hemisphere These conditions may be relevant to the considerations of establishing the

relationship between ophthalmic problems and dyslexia100

69

Only one study83 included this as part of the investigation Hoffman83 reported that one child

(093) of the population of learning disabled children examined had acute conjunctivitis

A major study on dyslexic subjects (although outside the cope of the present study but worth

citing) was conducted by Raju11 in 1997 She studied oculomotor control in dyslexia and

reported that sixty five percent of the normal readers exhibited normal oculomotor control as

compared to the eleven percent of dyslexics and that fifty four percent of the dyslexic

population exhibited deficiencies in both automaticity and oculomotor skills whilst twelve

percent of the control subjects displayed the same

70

LITERATURE REVIEW TABLE pages 70-72

71

72

73

211 Summary

Research into visual aspects of dyslexia has been undertaken mainly by optometrists and

ophthalmologists The main area of controversy is that ophthalmologists tend to deny any

relationship between vision and reading ability while optometrists assert that identifying and

correcting visual defects are an important part of managing dyslexia The findings of studies that

have investigated vision function in dyslexic population since the 1940s are inconclusive and

reflect the complexities associated with identifying the causes of dyslexic as well as its diagnosis

treatment and management

A range of study designs were used and the inclusion of a control group for example depended

on the objective of the study Some authors either compared the prevalence of vision defects in a

population of reading disabled group to the prevalence of similar vision defects in a group of

normal readers or only presented the prevalence of a particular visual condition in a group of

reading disabled children All the studies used convenience sampling methods at schools or by

including learners who were referred to their private practices the size being determined by the

number of dyslexic students available The main areas of vision functions studied were visual

acuity refraction binocular vision and ocular pathology The study findings were inconclusive

and the only vision variables consistently report ed across the studies was the association of

hyperopia with poor reading performance

The differences in results reported by the authors may be due to the methodological problems

such as differences in instrumentation and techniques failurecut-off criteria method of data

analysis lack of comparison group and subjectsrsquo selection from clinical population There was a

noticeable absence of studies on African children research has shown that African persons are

less at risk of myopia and more at risk for open-angle glaucoma than Caucasians There is

therefore scope for continued research among African children to determine whether there is any

relationship between dyslexia and vision

Research on dyslexia and vision that requires an interdisciplinary approach may yield different

results due to different approaches by different professionals However it is challenging to

74

design a study that would control for all variables possibly influencing reading ability and vision

function Reading performance is an extremely complex task and its measurement can be

influenced by many factors

Having reviewed the relevant literature on dyslexia and vision Chapter three will present the

research design the study population sampling procedure methods of data collection and

analysis

75

CHAPTER THREE METHODOLOGY

31 Introduction

This chapter will present the research design the study population sampling procedure

methods of data collection as well as method of data analysis

32 Research Design

The study was designed to provide empirical information to enable a comparison of the visual

characteristics of dyslexic children and normal readers A matched paired case-control

method was adopted Data was collected from both the dyslexic and control groups and

analysed

321 Sampling Design

Participants for both samples were selected using the convenient sampling method (based on

available subjects) and an optometric eye examination was conducted

322 Study Population and Participants

The study population comprised children attending a school for children with learning

difficulties from which the dyslexic children were selected to participate in the study while

the control group consisted of learners from a mainstream Durban school

i Dyslexic Group Participants for the study from the dyslexic group consisted of 31

African dyslexic children (the experimental group) selected from Khulangolwazi Special

School for children with learning difficulties in Clairwood South of Durban Psycho-

educational evaluationdiagnosis of dyslexia was not part of this study and was not

76

performed The school principal allowed access to all learnersrsquo file from which the

dyslexic learners where selected based on the school psychologistrsquos diagnosis Learners

attending this special school were referred from mainsream schools around Durban The

final diagnosis as to the type of learning difficulties was done by the school psychologist

None of the children were on any medication

The mean age for the dyslexic participants was 13 years and the learners were in grades

four through grade seven considering the fact that they were children that were reading two

grades behind their chronological age There were 15 boys and 16 girls Due to limited

number of learners who were classified as dyslexic it was difficult to recruit the targeted

number of one hundred participants for the study The subjects from both groups were

lsquoAfricanrsquo South Africans of African origin

ii Control Group Participants for the control group consisted of 31 children from a

mainstream school in Durban (Addington Primary School) The mean age of the children

was 11 years and 9 months A total of 15 boys and 16 girls were chosen ranging from grade

four to grade seven

323 Inclusion Criteria for the Dyslexic group

To be included in the study the subjects had to meet the following criteria

1 An average or above average intelligence quotient For this study an average was taken

as between 95 and above as recorded on the learners file by psychologist

2 Be two grades or more below grade equivalent in a mainstream school

3 The child has not been absent from school for more than 10 of the attendance days

The information on the learnersrsquo attendance was supplied by the school principal

324 Exclusion Criteria for the Dyslexic group

The following criteria resulted in children being excluded from the study

1 Presence of any emotional problems (information from the learnersrsquo file)

2 Presence of any systemic condition

77

3 Use of any systemic medication

The inclusion and exclusion criteria for the control group were similar to the experimental

group except that the children did not have any reading problems and were attending a

mainstream school

33 Ethical Clearance

The study protocol was approved by the then University of Durban-Westville research ethics

committee A letter of request for access to the schools was sent to the Department of

Education (Appendix B) Written informed consent (Appendix C) for access to the schools (for

both groups) was obtained from the Department of Education Written informed consents were

also obtained from the principals of the schools (mainstream and special school) (Appendix D)

Due to the difficulty in reaching the parents the school principals consented on behalf of the

parents for the learners to participate in the study The participants were fully informed of the

purpose of the study and accepted to co-operate with eye examination procedures

34 Testing Protocol

The testing was done first with the dyslexic learners and then at the mainstream school

35 Study Setting

The same testing arrangement was used for both groups A testing room was set up in one of

the offices provided by the school principal The distance acuity chart was placed at a point

six meters away from where the subjects sat and was used for the distance acuity chart and all

distance testing The examinations were done under room illumination except for some

procedures such as retinoscopy that required dim illumination resulting in the room being

dimmed

78

All testing was conducted in the morning as it was anticipated that better responses could be

obtained when the children were not tired Each school principal provided an assistant who

helped with various activities such as controlling room illumination The rationale and

technique for every procedure was fully explained to each participant and a trial reading was

taken to ensure that all instructions given were understood During the tests subjects were

asked (from time to time) if they were tired For any answer to the affirmative the testing was

discontinued and the child was allowed to have a break As a routine a break of 10 minutes

throughout the duration of entire procedure was allowed

The testing instruments were provided by the International Center for Eye Care Education

(ICEE) As the instruments were also being used by other researchers they were unavailable

on certain days resulting in some tests being postponed until a later date The data collection

took an average of two months per school All data were collected by the examiner alone and

each examination took an average of thirty minutes to complete

36 Data Collection Techniques

All tests were conducted in free space The data collection techniques used in the present

study was similar to techniques used in other studies 93100106-140 based on pediatric

populations (Table 31)

Table 31 Data Collection Tools and Measurement Outcome

InstrumentationTechnique Measurement Outcome

Bailey ndashLovie LogMAR Charts Visual Acuity

Royal Air Force (RAF) Rule NPC Amp of Accommodation

Welch Allyn Streak Retinoscope Objective Refraction

Trial Frame and Trial lenses Subjective Refraction

Cover Test - Occluder and Prism Bar Ocular Alignment

Maddox Rodpen torchPrism bar Heterophoria and Tropia

Maddox Wing Near Phoria

Prism Bar Heterophoria and Tropia

79

Pen torch PD rule Cornea reflex test-strabismus

Retinoscope and neutralizing lenses Accommodation Posture

Jackson cross cylinder Refine subjective findings

Trial Frame and neutralizing lenses Relative accommodation

+-2 flipper lenses Accommodation Facility

Direct Ophthalmoscope (Welch Allyn) Ocular Health

Prism Bars Fusional Reserves

361 Visual Acuity

Visual acuity is a measure of the acuteness or clearness of vision It assesses the ability of the

visual system to resolve detail which is dependent on the sharpness of the retinal focus within the

eye the sensitivity of the nervous elements and the interpretative faculty of the brain 106 Visual

acuity was assessed using the Logrithm of Minimun Angle of Resolution (LogMar) chart which

means that the letters change in size from line to line in equal steps of the log of the minimum

angle of resolution (MAR) Visual acuity was assessed monocularly and binocularly for both

distance and near The LogMar chart facilitates algebraic operations for statistical analysis and

allows for precise quantitative assessment of visual acuity It is recommended for use in research

studies in which visual acuity is a dependent variable106

Each subject was comfortably seated and the test was conducted with the examiner sitting in front

in such a way that the subjectrsquos view was not obstructed For monocular visual acuity the

subject was instructed to cover one eye with their right palm and to read the letters on the chart

This procedure was repeated binocularly and the test was done under normal room illumination 106 No subjects wore spectacles so visual acuity for aided vision was not indicated

362 Refractive Error

Refractive error was assessed objectively using the Streak Retinoscope (Welch Allyn) with a

+150D fogging lens (for an armrsquos length of approximately 67cm) with the subject fixating a

660 (to maintain fixation) optotype on the distance visual acuity chart As the subject focused

80

on the distance target on the chart the retinoscope light was passed across the subjects eye

(right hand used to scope right eye and left hand used to scope left eye) The motions

observed were neutralized with the appropriate lenses plus lenses for lsquowith motion and minus

lenses for against motion The subjective refraction was performed through monocular

fogging refined using the Jacksonrsquos cross cylinder followed by binocular balancing 106 The

retinoscopic results were used for analysis

Although retinoscopy was performed without cycloplegia as in other studies 395107108

cycloplegic refraction was not performed because the researcher (an optometrist) was not

licensed to use therapeutic drugs when the data was collected However it has to be

acknowledged that a non-cycloplegic examination among children has limitations among

those with hyperopia as the full extent of the hyperopia may be masked

363 Near Point of Convergence

The NPC was measured using the Royal Air force (RAF) rule The subject was instructed as

follows ldquoThis test measures your ability to turn your eyes in towards your nose Look

directly at the dot on middle of the line The image may appear to be blurring (not clear)

That is okay However if the dot becomes two say lsquotworsquo I will then pull the target away If

the dot becomes one again say lsquoonersquo rdquo The subject was asked to fixate on the dot on the

middle of the line as the target was advanced towards him The objective reading was taken at

the point when one eye loses fixation while the subjective reading was noted when the subject

reported the target to be double At a point when the subject reported double the target was

moved back until a point when the target was reported to be single again this was taken as the

recovery point The final break and recovery points recorded were the average of three tests

measurements The measurement was taken three times in order to detect fatigue which may

indicate poor convergence 111-113 The objective reading was taken for analysis

364 Strabismus

Strabismus was assessed using the Hirschberg test A penlight was held about 50cm from the

childs face in the mid plane and they were encouraged to fixate the light with both eyes open

81

The location of the corneal reflection on each cornea relative to the centre of the pupil was

then noted The displacement of the corneal reflection from the centre of the pupil (angle

lambda) was estimated in millimeters An ocular alignment was observed as a symmetric

displacement of the reflexes 114115 The examiner proceeded with the Kappa Test which is a

test of monocular fixation and is done while the child was monocularly viewing the examiners

penlight The penlight was maintained at 50 cm distance from the face The purpose of this

test was to determine the visual axis of each eye The examiner observed the location of the

corneal reflexes relative to the center of the pupil in the right eye and then in the left eye A

nasal displacement is signified by a (+) sign and a temporal displacement is signified by a (-)

sign with the amount quantified in millimeters away from center Again a normal location

will likely be either centered or +05 mm nasally displaced 114115

The interpretation of the magnitude and the type of deviation requires the data gathered from

both the Hirschberg tests The fixating eye is the eye in which the corneal reflex location is

the same on the Hirschberg and Kappa tests The strabismic eye is the eye in which there was

a difference The change in location from the Kappa tests as compared to the Hirschberg test

equals the amount and type of deviation Every 10 mm of corneal displacement is

approximately equivalent to 22 prism diopters A nasal change in location on Hirschberg

testing from the visual axis denoted by the Kappa test denotes an exotropia A temporal

change in location on Hirschberg testing from the visual axis denoted by the Kappa test

denotes an esotropia 114

365 Heterophoria

Ocular alignment was assessed using the cover test at six meters and 40cm for distance and

near respectively It is an objective technique and is very suitable for assessing ocular

alignment in children

For distance cover test the test target was a letter from the line above the subjectrsquos best visual

acuity of the worst eye The subject was advised that the muscle balance of their eyes was

assessed and that he should look steadily at the fixation target (an isolated letter on the chart)

at all times and ignore the occluder The coveruncover (unilateral) cover test was performed

82

to determine the presence of phoria or tropia 106114-115 To perform the unilateral cover test

the occluder was placed in front of the right eye held for approximately one to two seconds

and the left eye was observed for any movement If no tropia or phoria present the left eye

was covered and the right eye then observed Any movement observed was neutralized using

a loose prism bar in the corresponding base direction Base-in prism for exo deviation and

base-out for eso deviation

The alternating cover test was performed to assess the direction and magnitude of phoria or

tropia (it measures the total angle of deviation) 106114-115 The test conditions were the same as

the unilateral cover test The subject was comfortably seated and instructed to look at the

fixation target at distance The occluder was placed in front of the patientrsquos right eye held for

2-3 seconds and then quickly placed on the left eye The test was repeated several times with

the occluder held in front of each eye before quickly moving to the other eye while observing

the eye that was just uncovered Any deviation was neutralized accordingly 115 The Maddox

Wing was also used to assess near phoria under normal room illumination The technique was

assessed with the subject wearing his full subjective correction 115 The use of cover test for

the assessment of ocular alignment at both distance and near may have allowed for continuity

and standardization However the cover test and the Maddox wing were used by Evans et al 100 in the same study The subject was directed to look through the horizontal slits to view the

chart that comprised of a horizontal and vertical scale and a horizontal and vertical arrow The

construct of the Maddox wing is such that the right eye sees only the arrows while the left eye

sees only the scale It achieves dissociation by presenting independent objects to the patient

As the images are dissimilar the incentive to fusion is abolished and the eyes adopt the fusion

free position 115 The arrows are positioned at zero on the scales but through dissociation any

phoria will be indicated by an apparent movement of the arrow along the scale

To measure horizontal phoria the subject was asked ldquowhich white number does the white

arrow point tordquo The number on the scale represents the magnitude of the deviation and the

direction To measure vertical phoria the subject was asked to indicate which red number the

red arrow points to 115

83

366 Evaluation of Accommodation Functions

a Accommodation Posture The assessment of accommodation accuracy was performed at

the subjects habitual near distance using the monocular estimation method (MEM) The

MEM technique uses a specially made target with pictures or words suitable to the childs

reading level The card was attached to the retinoscope The procedure was performed

over subjects distance prescription and at his normal reading distance with enough light

to provide a comfortable reading vision Retinoscopy was performed on axis (right eye

before left eye) as the child reads the words or describes the picture on the card aloud As

the retinoscopic reflexes were observed an estimate of the magnitude of the motion was

made and trial lenses were then interposed briefly (approximately 05 seconds) to avoid

changing the childs accommodation status until no motion was observed The lens

power at which the motion was neutralized was taken as the lag or lead of

accommodation It is a lag of accommodation when the reflex is neutralized with

positive lenses while it is called lead of accommodation if neutralized with negative

lenses 112113116117

b Accommodation Facility this assesses the rate at which accommodation can be

stimulated and relaxed repeatedly during a specific time period It relates to the

individualrsquos ability to shift focus quickly and efficiently for varying distance The

procedure was performed with the best subjective correction in place monocularly and

binocularly but only the result for the binocular assessment was utilized for the data

analysis According to Wick and Hall 120 binocular accommodative facility testing may

not be a true accommodative facility measure such as monocular facility but clinically

binocular testing may be more useful because it gives a ldquoreal liferdquo assessment of

accommodation and binocular interactions of accommodation and vergence

The target used was letters on a 69 range on a near point card and at the subjectrsquos

habitual reading distance A lens flipper of 2 diopter lenses was used Letters and

pictures used were appropriate to the childs ability The subject was informed that he

would be shown two different lenses and that one pair causes the system to work while

84

the other pair relaxes it and was instructed to look at the materials as the lenses were

flipped Each time the print become clear and single he was to immediately report

clear The lenses were flipped and child should report clear when the material gets

clear To ensure that subject understood the instructions trial readings were first taken

The number of cycles completed per minute was recorded One cycle meaning clearing

both the plus and minus lens sides The test was done binocularly112-113118-120 At near

with no suppression control

c Amplitude of Accommodation The amplitude of accommodation was assessed by

Donderrsquos push ndash up method using a Royal Airforce (RAF) near point rule a rod with a

movable target and metrics as well as dioptric marking This procedure was performed

monocular and binocularly The subject was seated comfortably and instructed to read a

line of letters on the card that corresponded to a visual acuity of 69 and told to keep the

letters clear The target was slowly moved towards the child along the rule until the child

reported first sustained blur at which point the dioptric result was read off the Royal air

force rule Three readings were performed and an average taken This test was

conducted monocularly for each eye and then binocularly

As reported by Wick and Hall 120 the minimum amplitude of accommodation expected is

based on the subjects age and can be estimated from the Hofsetterrsquos formula

Minimum =15 - 025 (age)

Expected =185 - 03 (age)

Maximum = 25 - 04 (age)

The amplitude of accommodation was generally considered clinically important only

when it falls below the expected age minimum 120

d Relative Accommodation (RA) Relative accommodation tests are plus-to-blur (negative

relative accommodation) (NRA) and the minus-to-blur (positive relative accommodation)

(PRA) The relative accommodation tests assess patientsrsquo ability to increase and decrease

accommodation under binocular conditions when the total convergence demand is constant 106 It is also an indirect assessment of the vergence system since the vergence demand

85

remains constant while accommodative demand varies 121122

Relative accommodation was measured using the plus and minus lenses binocularly at

near with distance subjective results in place 122 In this procedure the child viewed a

reading target size letter size or picture of 66 size at the reading distance with the best

subjective correction in place Plus or minus lense powers was added in 025 diopter

steps in approximately 2 seconds interval121 to assess negative and positive

accommodation respectively This test was performed binocularly and the reading was

taken when the subject reported a sustained blur The relative accommodation is difficult

to assess with trial lenses121 The trial lenses were used to assess relative accommodation

in the present study The use of the phoropter was not possible considering the study

setting

367 Fusional Reserves

The term fusional reserve is used synonymously with vergence reserves and prism vergence

vergence amplitude 121123 or fusional amplitudes 111 Vergence ability is measured using

prisms placed in front of each eye The amount of prism is increased gradually to measure the

amount of fusional reserve the individual has in reserve to compensate for a phoria112 124 The

amount of base out prism required to produce diplopia is called positive fusional reserves or

positive fusional vergence while the amount of base in prism required to produce diplopia is

called the negative fusional reserves or negative fusional vergence 115

Positive fusional vergence (convergence) and negative fusional vergence (divergence) were

assessed using a prism bar in steps without suppression control although it has been reported

by Wesson et al 125 that when suppression is controlled the average vergence values will be

lower because the test is stopped when the suppression is detected that is if suppression is not

monitored the break is not detected until the stimulus is outside the suppression zone and a

higher vergence value may be obtained However

Scheiman et al 126 argued that the significance of such suppression in binocular individuals is

unknown

86

Assessing fusional reserves in children with loose prism bars may be a better technique

because it offers the possibility of viewing the eye movement objectively Secondly it

removes the restriction imposed by testing children behind the phoropter Finally the test

itself nearly duplicates the movement of the eyes under normal conditions allowing

convergence or divergence in discrete steps rather than as a smooth-pursuit type vergence

movement 125

The test was performed with the target at six meters (test target 69 letter line) and at 40 cm

(N5 text) The prism was placed in front of the right eye first Base-in was routinely assessed

first the rational for this sequence of testing being that the convergence response stimulated

during the base-out (or convergence) measurements may produce vergence adaptation ( a

fusional after-effect) which may temporarily bias the subsequent base in values in the base-out

direction 127 Typically there is no blur point for base -in vergence testing at distance This is

because at distance accommodation is already at a minimum and cannot relax beyond this

point 115

Base out prisms were used to measure positive fusional vergence while base in prisms were

used to measure negative vergence The subject was seated comfortably and instructed as

follows ldquoI am measuring the ability of your ayes to converge and diverge Watch the row of

letters Tell me when they first get blurry if they do Also tell me when they first become

two rows of letters and when they become one againPrime The prism was introduced over the

right eye lsquoDoublersquo was first demonstrated to the child by placing a 20 diopter prism over one

eye If blur was reported the prism power was increased until a break was reported The

prism power was then reduced until the subject reported that the rows of letters were single

The technique was then repeated for base out prism and then the test was repeated at near The

break and recovery points were determined subjectively from the childrsquos report of blur break

and recovery and objectively by observing the subjects eye movements The objective

findings were used for analysis

87

368 Ocular Health

The only test performed to evaluate ocular health was direct ophthalmoscopy

369 Vergence Facility

It was impossible to perform the vergence facility testing as the children could not fuse the

base in prisms So vergence facility data was not available for analysis The recovery point

of the base in reserves may provide useful information on the vergence facility According to

Bishop 111 ldquoblur and break points tend to reflect the quantity of fusion whereas the recovery

point indicates the quality of fusion that is the ease of change of fusional demands (facility)

and the ability to maintain fusion (stamina)

37 Statistical Analysis

The data entry was done by a staff optometrist who is skilled in statistical analysis The data

analysis was undertaken by the International Center for Eye Care Education statistician Data

was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 For comparison all data from both groups was subjected to a two-sample t-test (2-

tailed) (Table 32) Participants who did not meet the pass criteria for any of the test variables

were referred to their optometrist for further evaluation

Table 32 The Diagnostic Criteria for Each Test Variable

Variable Diagnostic Criteria

Visual Acuity 128-131 69

88

Refractive Errors 128-131

Hyperopia ge +075

Myopia ge -050

Astigmatism ge -075

Anisometropia ge 075 between the two eyes

Emmetropia lt - 050DS lt +100DS lt -075DCyl

Near Point of Convergence 132-134 ge 10cm

Accuracy of Accommodation 116117119120

Lag ge +075 Lead any minus finding

Accommodative Infacility 118119 gt 7cycles per minute in response to +2 -200 flipper testing

binocularly at near

Amplitude of Accommodation106119120

Binocular accommodative amplitude ge 2D below the expected

value for the patients age for minimum amplitude (using

Hofsetterrsquos formula) (15-14age)

Positive Relative Accommodation 106121122124

gt - 237DS

Negative Relative Accommodation 106121122124

gt + 2 DS

Phoria 106128

Distance

Near

gt 6 prism diopters exophoria or 4 prism diopters esophoria

gt 6 prism diopters exophoria or 4 prism diopters esophoria

Strabismus 131135 Manifest or intermittent manifest deviation of gt 2 prism

dioptres 135 or asymmetry in Purkinje reflex or compensating

fixation a misalignment of ge 25 degrees 131

Fusional Reserves LimitsDistance 119

Near 106121126

Base in Blur X Break 6 Recovery 4

Base out Blur 10 Break 16 Recovery 10

Base in Blur 14 4 Break12 5 Recovery 7 4

Base out Blur 22 8 Break 23 8 Recovery 16 6

89

38 SUMMARY

The study consisted of two groups of 31 participants from two schools in Durban The study

group was children with learning difficulties from which the dyslexic children were selected

and the control group was selected from a mainstream school The mean age for the dyslexic

participants were 13 years while the mean age of the children from the control group was 11

years and 9 months A convenience sampling method was adopted to select the study

participants due to the limited number of subjects to select from

The vision functions investigated may be divided into three broad areas of visual acuity and

refraction binocular vision and ocular pathology which corresponds with areas of vision

function investigated in the literature review Standard optometric testing procedures were

used to assess the vision variables

Visual acuity was assessed using the LogMAR chart at both distance and near Refractive

error was determined objectively using a streak retinoscope (without cycloplegia) while the

child fixated a 660 optotype at 6 meters with a fogging lens to control accommodation

The near point of convergence was measured using the Royal Air force rule (RAF) Ocular

alignment was assessed using the cover test at six meters and 40cm for distance and near

respectively For distance cover test the test target was a letter from the line above the

subjectrsquos best visual acuity of the worse eye The coveruncover (unilateral) cover test was

performed to determine the presence of a phoria or tropia The alternating cover test was

performed to assess the direction and magnitude of the phoria or tropia The test conditions

were the same as the unilateral cover test The Maddox Wing was also used to assess near

phoria under normal room illumination Strabismus was assessed using the Hirschberg test

with a penlight held about 50cm from the childs face in the mid plane and was encouraged to

fixate the light with both eyes open The location of the corneal reflection on each cornea

relative to the centre of the pupil was then noted

The different aspects of accommodation functions were also measured The assessment of

accommodation accuracy was performed at the subjects habitual near distance using the

90

monocular estimation method (MEM) The accommodation facility was assessed using a 2

diopter lens flipper with the best subjective correction in place The target used was letters on

a 69 range on a near point card and at the subjectrsquos habitual reading distance The amplitude

of accommodation was assessed by Donderrsquos push ndash up method using a Royal Airforce (RAF)

near point rule This procedure was performed monocular and binocularly with a 69 visual

acuity as the test target The relative accommodation was measured using the plus and minus

lenses The test target is a reading letter target size or picture of 66 size at the reading distance

with best the subjective correction in place This test was performed binocularly and the

reading was taken when the subject reported a sustained blur

The fusional reserves were assessed using prism bars in steps without suppression control

The test was performed at 6 meters (test target 69 letter line) and at 40 cm (N5 text) Base out

prisms were used to measure positive fusional vergence while base in prisms were used to

measure negative vergence Ocular health evaluation was assessed using the direct

ophthalmoscope

Data was analyzed using the Statistical Package for Social Sciences (SPSS) Means standard

deviation and ranges were calculated for descriptive and comparative purposes at 95

confidence interval The level of significance considered to support a hypothesis was taken as

P lt 005 To compare the data from both groups all data was subjected to a two sample t-test

(2-tailed) The methods used in this study were the same as those used for similar tests in the

international literature reviewed in Chapter Two and the sample size was larger than most of

the studies presented

Chapter Four will outline the study results with the comparative and descriptive statistical

analysis

91

CHAPTER FOUR RESULTS

41 Introduction

This chapter presents the study findings and the analysis of results obtained The data for the

prevalence of visual acuity refractive errors near point of convergence accommodation

functions and heterophoria are presented in histogram while the descriptive statistics are

presented in tables

42 Prevalence of Vision Defects

The prevalence is an indication of the subjects that have either achieved or did not achieve the

pass fail criteria and is not a normal distribution of a particular condition The prevalence rate

of the vision functions is expressed in percentages The other data is expressed as mean (M)

and standard deviation (SD) and calculated for descriptive purposes at 95 confidence

interval (CI)

If an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants would be expected

421 Visual Acuity (VA)

i Dyslexic Group In total 32 of subjects in the dyslexic group had visual acuity

less than 69 while 68 had visual acuity of 66 and 65 The distribution of those

subjects with VA le 69 is as follows 65 (2 subjects) had VAs of 69 65 (2

subjects) had VA of 612 65 (2 subjects) had VA of 615 3 (1 subject) had

VA of 630 3 (1 subject) each had VA of 638 648 660 respectively (Fig

41)

ii Control Group In total 32 of subjects in the control group had visual acuity

92

less than 69 while 68 had visual acuity of 66 The distribution is as follows 3

(1 subject) each had VA of 69 612 615 624 while 65 (2 subjects) had VA

of 630 638 and 648 respectively (Fig 41)

Fig 41 Prevalence of Visual Acuity

The mean visual acuity for the right eye for the dyslexic group was 017 031 and 000

024 for the right eye for the control group There was no statistically significant difference

between the two groups (p = 029) (Table 41) The mean visual acuity for the left eye for the

dyslexic group was 020 033 and 000 024 for the control group There was no

statistically significant difference between the groups (p = 023) (Table 41) LogMAR acuity

of 020 is equivalent to 69 in Snellenrsquos notation (Appendix E Visual acuity conversion table)

Table 41 Descriptive Statistics for Visual Acuity Variable Group N ( 95 CI) Minimum Maximum p

Mean (LogMAR)

SD

VAcuityRE

LE

DyslexicControl

DyslexicControl

3131

3131

017000

020000

031024

033024

000000

000000

010090

100090

029

023

422 Refractive Error

i Dyslexic Group Twenty three percent (23) of the dyslexic subjects had

refractive errors while seventy seven percent (77) were emmetropic (defined as

93

retinosocpic findings of less than 05 myopia less than +075 hyperopia or less than

075 astigmatism) (Fig 42) The distribution is as follows 65 (2 subjects) had

myopia 65 (2 subjects) had hyperopia (both latent) As cycloplegia was not

used in assessing refractive error an estimate of the diagnosis of latent hyperopia

was made based on a high difference between the retinoscopic findings and the

subjective refraction results In the event that the increased retinoscopic finding

(that is more plus) did not blur the subjectsrsquo vision latent hyperopia was assumed

About 10 (3 subjects) had hyperopic astigmatism 65 (2 subjects) had

amblyopia (defined as visual acuity in either eye of 2040 or a two-line difference

in acuity between the two eyes with no improvement with pinhole) 141 and 65 (2

subjects) had anisometropia (defined as a difference of ge 075 in sphere or cylinder

between the two eyes) (Fig 43)

ii Control Group Twenty two and a half percent (225) of the participants from

the control group had refractive errors classified as follows 65 (2 subjects)

had myopia 3 (1 subject) had hyperopia and 13 (4 subjects) had astigmatism

and 65 (2 subjects) had anisometropia No participant had amblyopia

Fig 42 Prevalence of Refractive Errors (total

01020304050607080

Myopia HyperopiaAstimatism (Hyperopic)Anisometropia Amblyopia Emmetropia

Prevalence

Dyslexic GroupControl Group

0

5

10

15

20

25

Prevalence

Refractive Errors (total)

Dyslexic GroupControl Group

94

Fig 43 Prevalence of the Types of Refractive Errors

Refraction data for two children from the dyslexic group who had cataracts could not be

obtained due to poor reflexes and were excluded from the analysis

The mean spherical equivalent (SE) refraction for the right eye for the dyslexic group was 086

098 and 070 103 for the control group was There was no statistically significant

differences between the two groups (p=066) The mean spherical equivalent refraction of the

left eye for the dyslexic group was 057 101 and 049 109 for the control group There

was no statistically significant difference between the two groups (p= 092) (Table 42)

Table 42 Descriptive Statistics for Refractive Error Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

RE (SE)

LE (SE)

DyslexicControl

DyslexicControl

2931

2931

086070

057049

098103

101109

-125-350

-150-350

500350

400350

066

092

423 Near Point of Convergence

i Dyslexic Group Thirty three percent (33 10 subjects) had NPC break points

(objective) of greater than or equal to 10cm while about sixty seven percent 67

(20 subjects) had NPCs of less than 10cm (Fig 44) One child complained of being

tired and did not participate in this procedure and was therefore excluded

ii Control Group Forty eight percent (48 15 subjects) had NPC break point

(objective) of greater than or equal to 10cm Fifty two percent (16 subjects) had

95

NPC less than 10cm (Fig 44) Data on NPC for one child from the dyslexic group

was lost

The mean NPC break was 890 503 for the dyslexic group and 1260 870 for the control

There was a statistically significant difference between the two groups (p = 0049) The mean

NPC recovery was 1400 588 for the dyslexics and 2200 820 for the control group

There was a statistically significant difference between the two groups (p = 006) (Table 43)

Table 43 Descriptive Statistics for NPC Break and Recovery Points Variable Group N (95 CI) Minimum Maximum P

Mean SD

NPC Break

NPC Recovery

DyslexicControl

DyslexicControl

3030

3030

8901260

14002200

503870

588820

500400

600800

26003400

28003800

049

006

424 Ocular Health

Six and a half percent (2 subjects) in the dyslexic group had cataracts (unclassified) on both

eyes No pathology was detected in the control group

425 Heterophoria

i Dyslexic Group No subject in the dyslexic group manifested with a phoria at

96

distance

At near 3 (1 subject) had an esophoria greater than or equal to 4 prism diopters

3 (1 subject) had an esophoria of 3 prism diopters 65 (2 subjects) had

exophorias of greater than or equal to 4 prism diopters 3 (1 subject) had an

exophoria greater than or equal to 8 prism dioptres and 65 (2 subjects) had

exophorias of 6 prism diopters (Fig 45)

ii Control Group At both distance and near no subject in the control group had

phoria of greater than 2 prism diopters

Fig 45 Prevalence of Phoria at Near (40cm) with Maddox Wing

Four children from each group could not complete the test as they left to attend lessons The

mean exophoria for the dyslexic group was 163 261 and 180 042 for the control group

There was no statistically significant difference between the two groups (p = 059) The

mean esophoria for the dyslexic group was 350 070 and 2 000 for the control group

There was no statistically significant difference between the two groups (p= 046) (Table 44)

No tropias were observed

Table 44 Descriptive Statistics for HeterophoriaVariable Group N ( 95 CI) Min Maxi P

97

Mean SD

At NearExophoria

Esophoria

DyslexicControl

DyslexicControl

2727

2727

163180

350200

261042

070000

000100

300200

1000200

400200

059

0469

426 Accommodation Functions

a Accommodation Posture As shown in Figure 46 the distribution of accommodation

lag is as follows

i Dyslexic Group

Thirty nine percent (11 subjects) had a lag of accommodation (defined as + 075 or

greater) three and a half percent (1 subject) had lead of accommodation (defined as

- 025 or greater) while fifty seven percent had normal accommodation posture

ii Control Group

Forty two percent (13 subjects) had lag of accommodation About eleven percent

(11) (3 subjects) had lead of accommodation while about thirty nine percent had

normal accommodation posture

Three children from the dyslexic group could not continue with this test as they had to

leave to attend class activities while three children from the control group sought

permission to be out of the testing room at that point These children were excluded

from the analysis for binocular accommodation lag

98

Fig 46 Prevalence of Accommodation Lag ( ge+075)

The mean accommodation lag for the right eye was 091 038 for the dyslexic group

and 092 057 for the right eye for the control group There was no statistically

significant differences between the two groups (p =083) The mean for the left eye for

the dyslexic group was 085 036 and 091 048 for the control group There was no

statistically significant difference between the two groups (p = 061) (Table 45)

Table 45 Descriptive Statistics for Accommodative PostureVariable Group N (95 CI) Minimum Maximum P

Mean SD

Accomm lag

RE

LE

DyslexicControl

Dyslexic Control

2828

2828

091092

085091

038057

036048

000-050

-050-050

200200

125200

083

061

b Accommodative Facility As shown in Figure 47 the distribution of accommodation

facility was as follows

i Dyslexic Group

Forty six percent (46 13 subjects) had normal accommodative facility (defined as

0

10

20

30

40

50

Prevalence

Bi no cu lar A ccom m od ation Lag

D y s lex ic G r o u p

C on t ro l G ro u p

99

greater than 7cpm) binocularly at near while (54) (15 subjects) had

accommodation facility less than or equal to 7cpm (Fig 47)

ii Control Group

Sixty seven percent (67) (18 subjects) had normal accommodative facility

(defined as greater than 7cpm) while thirty three percent (33) (9 subjects) had

accommodation facility less than 7cpm

Three children from the dyslexic group and four from the control did not complete the test

as they indicated that they were tired The mean binocular accommodation facility was

686 cpm 274 for the dyslexic group and 885 369 for the control group There was a

statistically significant difference between the two groups (p=0027) (Table 46)

Table 46 Descriptive Statistics for Binocular Accommodative FacilityVariable Group N ( 95 CI ) Minimum Maximum p

Mean SDAccommodativeFacility at near

DyslexicBE

ControlBE

28

27

686

885

274

369

2

2

12

21

002

7

c Amplitude of Accommodation Monocular amplitude of accommodation in both eyes

and for both groups did not differ significantly (not more than 050D) except for one

subject who had a difference of 4 diopters between the two eyes The amplitude of

100

accommodation for two subjects who had latent hyperopia in the dyslexic group was

relatively low (600DS and 800DS) as compared to age minimum amplitude of

accommodation of 1175DS The amplitude of accommodation for two participants who

had cataracts were excluded from the analysis

Only the data for the monocular amplitude was analyzed When accommodation

amplitude is assessed monocularly it measures the response for each eye individually

and is particularly important to determine whether a patient has accommodative

insufficiency 133 The monocular amplitude of accommodation has been used to assess

amplitude of accommodation function in several studies 86 88100102-103 The mean for the

right eye for the dyslexic group was 1198 234 and 1287 108 for the control group

There was no statistically significant difference between the two groups (p =007) (Table

47) The mean for the left eye for the dyslexic group was 1214 214 and 1287 116

for the control group There was no statistically significant difference between the two

groups (p =022) (Table 47)

Table 47 Descriptive Statistics for Amplitude of Accommodation Variable Eye Group N ( 95 CI ) Minimum Maximum p

Mean SDAmplit of Acomm

RE

LE

DyslexicControl

DyslexicControl

2931

2931

11981287

12141287

234108

215116

810

810

2015

2015

007

022

d Relative Accommodation (NRA PRA) As shown in Fig 48 the results for the relative

accommodation were as follows

For the PRA all subjects in both groups had PRAs of more than -200DS For the

negative relative accommodation (NRA) 89 of the dyslexic subjects had NRA greater

than +200DS while 96 of subjects from the control group had NRA greater than

+200DS

101

Fig 48 Prevalence of Relative Accommodation (NRA+2PRA-200)

Three children from the dyslexic group and five from the control group could not

continue with the test for the assessment of relative accommodation as they had to attend

important class activities

The mean PRA for the dyslexic group was -623 117 and -606 063 for the control

group There was no statistically significant difference between the two groups (p

=051) The mean NRA for the dyslexic group was 322 079 and 311 047 for the

control group There was no statistically significant difference between the two groups

(p =068)(Table 48) The findings for RA were higher than the published norm This

may be due to the process of addition of trial lenses

Table 48 Descriptive Statistics for Relative Accommodation Variable Group N ( 95 CI ) Minimum Maximum p

Mean SDPRA

N RA

DyslexicControl

DyslexicControl

2826

2826

-623-606

322311

117063

079047

-900-700

200200

-400-500

60045

051

068

427 Fusional Reserves

The children in both groups either could not report or understand blur so the result for break

102

and recovery was used in all analysis of vergence function

a Base-in Vergence Reserves at Distance two children from the dyslexic group could not

complete all aspects of the fusional reserves assessment because they were tired These

children were excluded in the analysis

The mean base-in to break for the dyslexic group was 1469 683 and 1600 350 for

the control group There was no statistically significant difference between the two

groups (p =046) The mean base in to recovery for the dyslexics was 1172 620 and

1280 317 for the control group There was no statistically significant difference

between the two groups (p =049) (Table 49)

Table 49 Descriptive Statistics for Base-in Vergence Reserves at distanceVariable Group N (95 CI) Minimum Maximum P

Mean SD

BI to Break (dist)

BI to Recovery (distance)

DyslexicControl

DyslexicsControl

2931

2931

14691600

11721280

683350

620317

410

28

4022

3520

046

049

b Base-in Vergence Reserve at Near The mean base in to break at near was 1185 514

for the dyslexics and 1283 313 for the control group There was no statistically

significant difference between the two groups (p=029) The mean base in to recovery was

872 478 for the dyslexics and 1032 335 for the control group There were no

statistically significant differences between the two groups (p=017) (Table 410)

Table 410 Descriptive Statistics for Base-in Vergence Reserves for NearVariable Group N (95 CI) Minimum Maximum P

Mean SDBI to Break (near)

BI to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

11851283

8721032

514313

478335

26

14

2518

2015

029

017

103

c Base-out (BO) Vergence Reserves at Distance Four children from the control

group could not give a report on the recovery point Two children from the dyslexic group

could not complete the test The mean base out to break at distance for the dyslexic group

was 2706 925 and 2416 975 for the control group There was no statistically

significant difference between the two groups (p=024) The mean BO to recovery for the

dyslexic group was 1876 796 and 17 693 for the control group There was no

statistically significant difference between the two groups (p= 040) (Table 411)

Table 411 Descriptive Statistics for Base-out Vergence Reserves at Distance Variable Group N ( 95 CI) Minimum Maximum p

Mean SD

BO to Break(Distance)

BO to Recovery(dist)

DyslexicControl

DyslexicsControl

2931

2927

27062416

18761700

925975

796693

10001000

400600

0004000

35003500

024

040

d Base-out Vergence Reserves at Near The mean base-out to break at near was 2160

1162 for the dyslexic group and 2109 842 for the control group There was no

statistically significant difference between the two groups (p =084) The mean base-out

to recovery at near for the dyslexic group was 1335 745 and 1555 625 for the

control group There was no statistically significant difference between the two groups

(p =016) (Table 412) The comparative and descriptive statistsics for all variables is

shown in Table 413

Table 412 Descriptive statistics for base-out vergences at near Variable Group N (95 CI) Minimum Maximum P

Mean SD

BO to Break (near)

BO to Recovery (near)

DyslexicControl

DyslexicControl

2931

2931

21602109

13351555

1162842

745625

8001000

600800

4040

3530

084

016

104

Table 413 Comparative Descriptive Statistics for Dyslexic and Control groups

Variable Dyslexics( 95 CI)

Control( 95 CI)

N Mean SD N Mean SD PVisual Acuity

RE 31 017 031 31 000 024 029LE 31 020 033 31 000 024 023

Refractive ErrorRE 29 086 098 31 070 103 066LE 29 057 101 31 049 109 009

NPC Break 30 890 503 31 1260 870 049NPC Recovery 30 14 588 31 2200 820 006

Heterophoria (Near)Exo 27 163 261 27 180 042 059Eso 27 350 070 27 200 000 046

PRA 28 -623 117 26 -606 063 051NRA 28 322 079 26 311 047 068

Amp of AccommodationRE 29 1198 234 31 1287 108 007LE 29 1214 215 31 1287 116 022

Accom Facility (bin) 28 686 274 27 885 369 003

Accomm LagRE 28 091 038 31 092 057 083LE 28 085 036 31 091 048 061

BI Break (Dist) 29 1469 683 31 1600 350 046BI Recovery (Dist) 29 1172 620 31 1280 317 049

BI Break (Near) 29 1185 514 31 1283 313 029BI Recovery (Near) 29 872 478 31 1032 335 017

BO Break (Distance) 29 2706 925 31 2416 975 024BO Recovery (Distance) 29 1876 796 31 1700 693 040

BO Break (Near) 29 2160 1162 31 2109 842 084BO Recovery (Near) 29 1335 7 45 31 1555 625 016

105

Summary Statistical tests were performed on the data to ensure scientific validity and were presented for

comparative and descriptive purposes The prevalence of visual acuity was similar between the

dyslexic and control groups and there was no statistically significant difference between the two

groups (p = 029 right eye 023 left eye)

The prevalence of total refractive errors was similar between the two groups and there was no

statistically significant difference between the two groups (p = 066 right eye and 092 left

eye) Hyperopia was slightly more prevalent in the dyslexic group than in the control group

The prevalence of myopia was the same in both groups Astigmatism was more prevalent in

the control group than in the dyslexic group The prevalence of anisometropia was the same in

both groups Amblyopia was more prevalent in the dyslexic group compared to the control

group

The control group had a higher near point of convergence break and recovery points than the

dyslexic group The NPC showed a statistically significant difference between the two

groups NPC break (p =0049) and NPC recovery (p =0006) The prevalence of exophoria at

near was higher in the dyslexic than the control group There was no statistically significant

difference between the two groups (p= 059) The prevalence of esophoria in the dyslexic

group was higher than that found in the control group but there was no statistically significant

difference (p =046)

The prevalence of relative accommodation was similar in both groups and there was no

statistically significant difference between the groups The dyslexic group had a reduced

amplitude of accommodation compared to the control group and there was no statistically

significant difference between the two groups (p=007 right eye andp = 022 left eye) The

dyslexic subjects performed significantly worse than the control subjects in accommodative

facility function and there was a significant difference between the two groups (p =0027)

The dyslexic subjects had a lower accommodation lag than the subjects from the control

group There were no statistically significant differences between the two groups right eye (p

= 083) and left eye (p = 060)

At near the negative fusional vergence (base in vergences)) was reduced in the dyslexics

compared to the control group while the positive fusional vergence (base out vergences) was

similar for both groups There was no statistically significant difference between the groups

106

(p = 021 for BI break at near) and (p = 084 for BO break at near) Having analyzed the

results Chapter Five will discuss the findings in the context of the international studies

reviewed in the literature

107

CHAPTER FIVE DISCUSSION

51 Introduction

A review of the literature reveals that studies 2356878990 conducted on dyslexic children have

investigated the prevalence of vision defects in the dyslexic population and examined whether

such vision anomalies are correlated with dyslexia by comparing vision characteristics in

dyslexic subjects to normal readers

The aim of this study was to determine the prevalence of vision defects in a South African

population of the dyslexic school children and investigate the relationship between vision and

dyslexia by comparing the vision characteristics of the dyslexic to the control group of learners

from a mainstream school The proposed null hypothesis was that there was no statistically

significant difference between the means of the vision functions in the dyslexic population

compared to the control group A discussion of the results of this study is presented

52 Prevalence of Vision Defects

521 Visual Acuity

The 32 prevalence of visual acuity worse than 69 was the same for both groups A reduced

visual acuity could be due to ocular abnormality (such as cataract) or due to normal refractive

error differences in the population Visual acuity defects due to ocular diseases are usually not

common in paediatric populations142 but tend to be related to refractive error changes in the

population143 This relates more in the present study as the prevalence of defects of visual acuity

and the total refractive errors were similar Consequently in relating visual acuity to refractive

errors myopia astigmatism and hyperopia may reduce distance or near visual acuity

108

Visual acuity testing is widely performed in screening refraction and monitoring of disease

progression It is also used in both basic and clinical vision research as a way to characterize

participants visual resolving capacities144 Specific to the present study visual acuity testing

helps detect visual problems at the distances at which most school learning activities occur and

gives useful preliminary information on how good or poor a childrsquos vision is reduced distance or

near visual acuity gives an indication of poor vision that will necessitate further examination

However in assessing how visual acuity relates to reading performance it is important to note

that visual acuity as measured with the traditional letter identification only assesses the ability to

discern letters on the visual acuity charts and does not assess dynamic vision behaviour

binocular eye control or ocular stress and so may not necessarily mean normal vision

Furthermore normal visual acuity may not necessarily mean normal vision since some people

may have other vision defects such as color vision or reduced contrast or inability to track fast-

moving objects and still have normal visual acuity The reason visual acuity is very widely used

is that it is a test that corresponds very well with the normal daily activities a person can handle

and evaluates their impairment to do them 145

In relation to previous studies the visual acuity findings in this study are similar to reports by

Buzzelli 6 Goulandris et al 90 and Metsing and Ferreira 103 On the contrary Hoffman 83 found

a lower prevalence (1028) of visual acuity worse than 69 in a group of learning disabled

children In contrast to the present study however the mean age of subjects in Hoffmanrsquos study

was 8 years 6 months while the mean age in the present study was 13 for dyslexics and 11years 9

months for the control group Grnlund et al 140 documented that age must be taken into

consideration when describing and comparing VA in different populationsbecause VA develops

from birth to adolescence A lower prevalence (10) of visual acuity worse than 69 was also

reported by Sherman84 Latvala et al 3 reported a slightly worse distance acuity in the dyslexic

group than in the control group but noted no statistically significant difference Evans et al 5

found that dyslexic groups had a significantly worse visual acuity than the control group

(Binocular distance VA p =00164 and binocular near VA p =00018) Similarly Ygge et al2

reported that the subjects from the control group had a better visual acuity than the dyslexic

group at both distance and near and that the results showed statistically significant differences

(distance at p =003) (near at p=0005) Bucci et al 89 and Kapoula et al 87 reported the

109

prevalence of visual acuity of all participants in their study to be normal A study conducted by

Grisham et al 86 lacked a control group but it was reported that 568 of the participants had 66

acuity in both eyes (compared to 68 in dyslexic group in the present study) 172 had 69 or

worse (compared to a total of 315 who had visual acuity of 69 and worse in the present study)

The mean age of the participants in the study by Grisham et al 86 was 154 years while the mean

age of the participants in the dyslexic group in the present study was 13 years

The factors which may affect the outcome of visual acuity measurements that may lead to

variations in results reported by different authors include size position and distribution of retinal

mosaics (retinal photoreceptors) 143 optical aberration pupil size clarity of the optical media

age method and type of chart used contrast isolated or multiple letters state of accommodation

illumination as well as the criteria used to define visual acuity114 140 Psychological factors

affecting visual acuity testing results include blur interpretation fatigue and malingering 143

These factors may be difficult to control and could constitute confounding variables which may

lead to variations in visual acuity results reported by different authors

522 Refractive Error

The prevalence of uncorrected refractive error of 23 found in the dyslexic group was similar to

the 225 found in the control group and there was no statistically significant difference between

both groups (p = 066 right eye p = 092 left eye) This finding may mean that the dyslexic

subjects are not at more risk of a particular refractive anomaly compared to participants from the

control group as the dyslelxic gourp has similar distribution of refractive errors

Some studies 5838498103146 did not classify refractive error according to the types but presented

the results for the total refractive error On this basis the prevalence of total refractive error

(23 for dyslexic and 225 for the control group) in the present study is similar to reports by

other studies Hoffman83 reported a 215 prevalence of refractive error in a study of the

incidence of vision difficulties in learning disabled children Although there was a marked

difference in the sample size in both studies the similarities in the findings may be because as

with the present study Hoffmans study was based on a population of learners in a special school

110

referred for optometric care by educators psychologists and reading specialists

Similar to the present study Evans et al 5 and Ygge et al 2 reported that the prevalence of

refractive errors in the dyslexic and control groups was similar Metsing and Ferreira103 found

no statistically significant difference between the learning disabled and the mainstream

groups in their study Contrasting findings were reported by Sucher and Stewart 98 who found

the prevalence of refractive errors to be 94 for the control group although the 208 found in

their dyslexic group is similar to the 23 found in the present study Wesson 146 reported the

prevalence of refractive error in children with reading dysfunction to be 38 while the

prevalence of refractive errors was 16 in the study by Sherman 84 In the study by Alvarez and

Puell 88 the prevalence of refractive error was 424 in the dyslexic group while 415 of the

control group had refractive error When compared to the present study the higher prevalence of

refractive error in the dyslexic group can be accounted for by the relatively high prevalence of

hyperopia (287) and in the control group the higher prevalence of refractive error can be

accounted for by the high prevalence of myopia (194) It is unclear why the prevalence of

hyperopia was as high as no information was given on how refractive error was assessed

Statistically for the present study the mean refractive errors for the dyslexic group were right

eye 086 098 left eye 057 1013 and right eye 070 103 left eye 049 109 for the

control group There was no statistically significant difference (RE p =066 LE p = 092)

Evans et al 91 reported a mean spherical refractive error of 077DS (right and left eye similar) In

the study by Alvarez and Puell 88 the refractive error results (descriptive statistics) reported

were as follows for the poor readers mean spherical equivalent refractive errors for the right and

left eye were 020 06 and 020 06 respectively In the control group mean spherical

equivalent refractive errors for the right and left eye were - 020 08 and - 014 08

respectively The descriptive statistics on refractive errors was not reported in the studies 3 93

cited earlier which made it impossible to compare the statistical findings with the present study

In the present study about 68 of participants from both groups were emmetropic (defined as lt -

050DS +100DS and 075D cyl) while 655 of the dyslexic children and 645 of children

from the control group were emmetropic in the study by Alvarez and Puell 88 Only the study by

111

Alvarez and Puell 88 reported on emmetropia

Refractive errors are among the leading causes of visual impairment worldwide and are

responsible for high rates of visual impairment and blindness in certain areas School children

are considered a high risk group because uncorrected refractive errors can affect their learning

abilities as well as their physical and mental development147

Several factors may lead to variations in results found in different studies This include the type

of population studied (clinical or non-clinical) sampling method (convenience or cluster)

classification criteria examiner bias and more specifically the use of cycloplegia in the

assessment of refractive errors

The following studies2 3 8895 categorized refractive errors as hyperopia myopia astigmatism

anisometropia or amblyopia

a Hyperopia In the present study hyperopia appeared slightly more prevalent (65) among

the dyslexic group than in the control group (3) This result agrees with the report from

other studies 93 95 and Alvarez and Puell88 who reported a much higher prevalence (287)

for the dyslexic group compared to 161 of the control group Helveston85 found no

difference in the prevalence of hyperopia between the population of normal and poor

readers

Hyperopia is the refractive error that is consistently reported 3939598 to be associated with

reading difficulties so it was expected that one will find the higher prevalence of hyperopia

in the present study

In the present study two children had latent hyperopia (based on the assumption that

increased plus did not blur the distance vision) The full magnitude of the hyperopic

findings could not be estimated because cycloplegia was not used Therefore it is possible

that the prevalence of hyperopia may have been under-estimated Williams et al 148 studied

the ldquoprevalence and risk factors for common vision problems in childrenrdquo and concluded

112

that the 48 prevalence of hyperopia in their study was ldquoan underestimate as data was

collected without cycloplegiardquo Borghi and Rouse (cited by Scharre and Cree)149

determined that on an average 063D more plus was identified by cycloplegic refraction

In relating hyperopia to reading difficulties simple to moderate hyperopia may not cause

constant blur at a distance or near point but the extra accommodative effort produces

asthenopic symptoms of intermittent blur headache fatigue and loss of concentration and

inattention in some patients which may be mistaken for short attention span Uncorrected

hyperopia is associated with esophoria at near point which can stress the fusional vergence

systems that hold the eyes in correct alignment If the hyperopia and esophoria is excessive

an accommodative esotropia can result 81150 This may result in difficulty in reading

b Myopia The 65 prevalence of myopia was the same in both groups This result is similar

to an equal prevalance of myopia reported by Eames 95 (4 in both groups) Alvarez and

Puell 88 reported that 57 of dyslexics were myopic while 194 of the control group was

myopic The prevalence of myopia was higher in studies by Rosner and Rosner 93 (54

non- learning disabled and 19 learning disabled) and lower than the prevalence reported

by Grisham and Simons150 In contrast to the present study subjects from the study by

Rosner and Rosner 93 were patients of an eye clinic where the two main reasons for referral

were reduced visual acuity and school learning problems while subjects for the present

study were referred to the special school mainly due to low academic performance which

may not necessarily be vision- related The subjects were chosen for the study irrespective

of whether they complained of a visual impairment or not whereas in the study by Rosner

and Rosner 93 study subjects had been identified as having a visual impairment Therefore

the higher prevalence may have been from the selection protocol used by Rosner and

Rosner 93 in their study

The prevalence of myopia was not reported in several studies 3568498146 referenced but was

presented as ldquorefractive errorsrdquo (was not classified as myopia hyperopia or astigmatism)

The onset and progression of myopia may be influenced by factors such as environment

nutrition genetic predisposition premature and low birth weight the effect of close work

113

racial and cultural factors differences in pupil size and illumination143151 Variations in the

prevalence of myopia reported by different authors may be related to the variation in the

above-mentioned factors

c Astigmatism

The prevalence (13) of astigmatism in the control group was higher than in the dyslexic

group (10) and there was no statistically significant difference between the two groups

Similarly Alvarez and Puell 88 found a higher prevalence of astigmatism in the control

group compared to the dyslexic group astigmatism was detected in 161 from the control

group compared to 8 and 92 right and left eyes respectively from the dyslexic group

In contrast Latvala et al 3 reported that 36 in the dyslexic group had astigmatism and

none from the control group had astigmatism Ygge et al 2 reported that the prevalence of

astigmatism was higher in the dyslexic group (28 left eye 25 ) compared to the control

groups (right eye 183 left eye 243) although there was no statistically significant

difference between the groups (p=025)

Rosner and Rosner93 reported that 30 of non-learning disabled and 27 of learning

disabled participants had astigmatism which differed markedly from the findings in the

present study despite the similarities in the study population Eames95 found an equal

prevalence of astigmatism (7) between the two groups studied

Astigmatism affects vision in different ways Clinically astigmatism over 150 diopters can

often cause severe eye strain and interfere with reading and even lesser degrees of

astigmatism can be symptomatic in some patients 81

Naidoo et al 157 studied refractive error and visual impairment in African school children in

South Africa Although the study was conducted specifically on mainstream school

children it relates to the present study as it provided useful information on the visual

characteristics of school children in South Africa and forms a basis for comparison since all

the studies reviewed in the present study were conducted on Caucasian population except

114

for the study by Metsing and Ferreira 103 which did not provide much information on

refractive errors in the their study of learning disabled children in Johannesburg South

Africa

Naidoo et al 157 reported a prevalence of hyperopia of 18 myopia of 29 astigmatism

of 67 and 68 right eye and left eye respectively In comparison the prevalence of

refractive error for the control group in the present study was hyperopia 3 myopia 65

and astigmatism 13 The refractive error trend (astigmatism being higher in prevalence

than myopia and hyperopia) was similar between the present study and the study by Naidoo

et al 157 despite the difference in prevalence reported Given that ethnic origins culture

socio-economic class are comparable between both studies the difference in prevalence

may (partly) have been due to different sampling methods (cluster versus convenience) and

the use of cycloplegia in their study The use of cycloplegia has been reported to yield

spherical aberrations and unpredictable errors due to associated mydriasis 110

It has also been documented 158 that the use of cycloplegia yields more positive retinoscopic

findings although this did not seem to apply in this comparison as the prevalence of

hyperopia in the present study was higher than that reported by Naidoo et al 157 A possible

explanation for the difference in prevalence may be related to the different criteria used to

define hyperopia Bennet et al 159 stated that the degree of ametropia ldquohellip considered to be

within normal limits in a particular study will drastically affect the prevalencerdquo Similar

opinion was noted by Gronlund et al 140 who stated that ldquothe choice of definition will of

course have a great impact on prevalence In their study on Swedish children aged 4-15

years Gronlund et al 140 defined astigmatism as ge 075 and stated that the prevalence of

astigmatism would have been 21 (instead of 32) had they defined astigmatism as ge

100 Furthermore Grisham et al 86 in their ldquoStudy of poor readers in high schoolrdquo noted

that helliprdquoif one selected less stringent criteria then the proportion of students with any given

dysfunction would decreaserdquo In the present study the criteria used to classify hyperopia

were ge +100D while Naidoo et al 157 used ge +200DS cut- off

115

d Anisometropia Anisometropia may be defined as a difference of 075 (sphere or cyl) or

more between the two eyes and this definition was the criteria used in this study 129

Anisometropia is of great clinical interest because of its intimate association with

strabismus and amblyopia141 152

The 65 prevalence of anisometropia was similar in both groups This is comparable to

results reported by Latvala et al 3 where the prevalence of anisometropia was 36 (2

subjects) for the dyslexic group and 6 (3 subjects) for the control group but contrasts with

findings by Eames 95 who reported the prevalence of anisometropia to be 13 for poor

readers and 6 for the control group while Ygge et al 2 found a higher prevalence of

anisometropia in the control group (158) than in the dyslexic group (94)

In anisometropia the difference in refraction as well as the refractive error causes the image

to be out of focus on one retina blunting the development of the visual pathway in the

affected eye 141 The fovea of an anisometropic eye receives images from the same visual

object however the images from the more myopic or hyperopic eye are out of focus 153 It

appears that anisometropia is a major cause of amblyopia for at least one third of all

amblyopias 131 and anisometropia is considered to be a causal factor in the pathogenesis of

amblyopia and strabismus It is also estimated that 6 to 38 of all cases of amblyopia are

caused by anisometropia without strabismus whereas in about 12 to 18 of the children

with strabismus this is accompanied by anisometropia 154

According to Grosvenor 114 uncorrected anisometropia even of a moderate amount may

induce eyestrain because it is impossible for the accommodation mechanism to maintain

clear images on the retina at the same time On the other hand large amounts of

anisometropia (2DS or more) may not cause symptoms as no effort is made to maintain

single binocular vision 114

Rutstein and Corliss155 investigated the relationship between anisometropia amblyopia and

binocularity and concluded that higher degrees of anisometropia generally cause deeper

116

amblyopia and poorer levels of binocularity for hyperopes but not for myopes and stated

that ldquoAlthough uncommon small amounts of anisometropia can cause moderate

amblyopiardquo However the results of the present study failed to support this claims as none

of the subjects had amblyopia resulting from anisometropia

In anisometropia the displacement or distortion of the image prevents the development of

fine visual perception in the occipital cortex and puts the child at risk for developing

amblyopia 141 Anisometropia causes poor reading skills probably through the mechanism

of poor sensory and motor fusion rather than reduced visual acuity It degrades binocular

coordination and consequently reduces visual comfort and efficiency if the binocular

coordination is under stress 150

e Amblyopia Two subjects (65) in the dyslexic group had ambyopia due to cataracts and

none from the control group had amblyopia This result may be comparable to studies by

Latvala et al 3 who reported 36 (2 subjects) for the dyslexic group and that of Rosner and

Rosner 93 with a prevalence of 4 for the non-learning disabled group and 3 for the

learning disabled group Cataract is a major cause of amblyopia (deprivation ambyopia) by

causing an impediment to the visual axis 141 Other conditions that may lead to amblyopia

include strabismus (which causes the images on the retinas to be dissimilar) and uncorrected

refractive errors (especially as in anisometropia) Anisometropia is related to amblyopia

Amblyopia is a major public health problem It is the most common cause of monocular

vision loss in children and young adults Early recognition and prompt referral are crucial

especially during infancy and childhood to prevent permanent loss of

vision 141

523 Heterophoria

Heterophoria is the relative deviations of the visual axes when the eyes are dissociated 160 Some

degrees of heterophoria are considered normal for persons with normal binocular vision

Approximately 1-2 prism dioptres of esophoria or 1- 4 diopters of exophoria at distance should

117

be considered to be within normal limits161 At near 3-6 prism dioptres of exophoria is

considered normal (physiological exophoria) 161 A normal healthy eye is usually able to

overcome these small deviations and so it is described as being compensated161 However

esophoria is much less compatible with comfortable vision than is exophoria Any amount of

esophoria in a patient with visually related symptoms may be problematic 162

In the present study all subjects were orthophoric at distance This report corroborates reports by

other authors Mathebula et al 163 studied heterophoria in a South African population of school

children (mean age 10 16 years) and reported that the mean distance horizontal heterophoria

showed orthophoria Similarly Evans et al 100 reported that all subjects examined ldquowere

orthophoric at distance with cover testrdquo According to Dowley160 ldquothe high prevalence of

orthophoria is realrdquo and that ldquothe high prevalence of distance orthophoriahellip may be due to the

coordinating influences of the same mechanisms that subserves prism adaptationrdquo A similar

view was expressed by Walline et al 164 who studied the ldquodevelopment of phoria in childrenrdquo and

stated that ldquonearly all of our subjects were orthophoric at distancerdquo

The major findings in heterophoria was a 95 prevalence of exophoria at near which was more

in the dyslexic subjects than in the control group with no exophoria There was no statistically

significant difference between the two groups (p= 059) At near 3 (1 subject) of the dyslexic

group had an esophoria greater than or equal to four prism diopters An interpretation of the

above finding is that the higher prevalance of heterophoria may mean that the dyslexic subjects

may be more uncomfortable when doing near work than the nroaml readers A possible

explanation may be that according to von Noorden (cited by Kommerell et al)165 heterophoria

typically causes asthenopia Patients with asthenopic symptoms (feeling of heaviness dryness

and soreness of the eyes pain in and around the eye frontal and occipital headache and the eyes

are easily fatigued) often have an aversion to reading and studying Typically such complaints

tend to be less severe or to disappear when patients do not use their eyes in close work165 It has

been documented that decompensated heterphoria (heterophoria accompanied by symptoms) can

be due to poor fusional reserves problems with sensory fusion (such as in anisometropia) or may

be due to an unusually large phoria which may be due to anatomical reasons or uncorrected

hyperopia 161166 The presence of phoria could contribute to visual and attention abnormalities

118

noted among children with reading difficulties making near point visual activities more

strenuous

Nearpoint esophoria has been reported to be associated with childhood myopia progression 164167

In the present study the only subject that had four prism diopters esophoria at near in the

dyslexic group had +125 hyperopia while the subject that had 3 prism diopters esophoria had -

150 DS myopia However due to the small sample size in the present study it is difficult to

draw any conclusion on the association between myopia and esophoria

The findings on heterophoria as in other aspects of vision functions in the dyslexic populations

reviewed are mixed However similar to the present study Latvala et al 3 assessed near

heterophoria using the Maddox Wing and reported a higher prevalence of near exophoria in the

dyslexics than in the control group Similarly Kiely et al 12 documented that dyslexics tend to

be more exophoric than normal readers Evans et al 101 found a higher prevalence of lateral

phoria in learning disabled subjects compared to normal readers In contrast to the above

findings Sucher and Stewart 98 assessed phoria using the cover test (testing distance was not

indicated in the results) but found a slightly higher prevalence of phoria in the control group than

in the dyslexics Evans et al 100 assessed near horizontal phoria using the Maddox Wing as in

present study but found no difference in near horizontal phoria between the dyslexic and control

groups Bucci et al 89 reported that there was no difference in phoria results at both distance and

near between the dyslexic and control groups Metsing and Ferreira 103 found no statistically

significant difference between the learning disabled and the mainstream groups on

heterophoriaThe use of the cover tests for both distance and near to assess heterophoria may have allowed for

standardization in technique but may not have implied that more valid results will be obtained

The cover test and the Maddox Wing have been used simultaneously to assess heterophoria in

other studies3 100 The possible sources of variations in results reported by different investigators

include classification criteria and poor technique 168

524 Near Point of Convergence

119

In the present study the prevalence of a remote (greater than 10cm) near point of convergence

was higher in the control group with 48 than in the dyslexic group with a prevalence of 33

There was a statistically significant difference in the near point of convergence break (p=0049)

and recovery (p = 0006) between the two groups

It may be that children who do not have reading difficulties tend to read more often than children

who experience difficulty to read This is because with increasing ability to read there is likely

to be more demand on accommodation and convergence resulting in near point stress as well as

esophoria The normal readers use and exercise the accommodation and convergence system

more frequently According to Owens and Wolf-Kelly 169 near work has long been considered a

potential source of visual problems stating that in their study reading ordinary text at a near

distance for about one hour induced significant changes in the resting postures of both

accommodation and vergences The authors indicated that ldquonear work induces a recession of the

near point of accommodation or vergencesrdquo

The near point of convergence break is associated with changes in inter-pupillary distance with

age As inter-pupillary distance widens with physical growth the amount of convergence in

prism diopters increases for a given distance from the subject 132 Chen et al 170 studied near point

of convergence in children aged 1-17 years and reported that an increasing incidence of remote

near point of convergence with increasing age in their study might be due to the near work

demands of primary school which might create a different level of near point stress than the near

work conditions in pre- primary school years

Similar to the findings in the present study Evans et al 91 reported a statistically significant

relationship between NPC and reading retardation (p=0019) Contrasting findings were reported

by other authors Bedwell et al 171 studied visual and ocular anomalies in relation to reading

difficulties and reported that 466 of the group of ldquogood readersrdquo had abnormal near point of

convergence compared to 58 of ldquopoor readersrdquo Similarly Bucci et al 89 and Kapoula et al 87

reported that the near point of convergence was significantly more remote in dyslexics than the

control In the study by Latvala et al 3 1275 of the dyslexic group and only 2 from the

120

control group had a near point of convergence of worse than 8cm A statistically significant

difference between the two groups was reported (p=00385) Metsing and Ferreira 103 found

no statistically significant difference between the learning disabled and the mainstream

groups on the near point of convergence function In a study that lacked control subjects

Grisham et al 86 reported a lower prevalence (154) of NPC (9cm or further) in a group of high

school poor readers The authors measured NPC three times on each subject in other to detect

fatigue but stated that it should therefore be noted that fatigue may be a problem for poor

readers even if they are able to converge at 8 cm if they tire easily then the ability to sustain

convergence during longer periods of reading may suffer

In the present study the measurement of the near point of convergence was repeated three times

on each subject in order to detect visual fatigue that the children experience everyday This

approach was documented by other authors 868789 111-113 Some authors111-113 have recommended

the measurement of the near point of convergence several times in order to detect fatigue which

is often indicative of poor convergence fusion system which may affect distract a child from

reading 172

525 Accommodation Functions

a Accommodation Facility Accommodative facility assesses the rate at which

accommodation can be stimulated and inhibited repeatedly during a specific time period It

relates to the individualrsquos ability to shift focus quickly and efficiently for varying distances

and is extensively used in the reading process 6

Only the result for the binocular accommodative facility was recorded and was used for analysis

in the present study Siderov and Johnston 174 noted that monocular accommodation

measurements provides a direct evaluation of the dynamics of accommodative response while

binocular testing of accommodative facility provides similar information but also reflects the

interactive nature of the relationship between accommodation and vergences

For the dyslexic group 54 (15 subjects) had inefficient accommodation facility while only 33

121

(9 subjects) of the control group had inefficient accommodative facility Statistically the control

group had a better accommodative facility (885cpm 369) than the dyslexics (686cpm 274)

and there was a statistically significant difference between the two groups (p =0027) It has

been documented 100 that an abnormal accommodative facility could imply difficulty in changing

focus from far to near and subsequently lead to a lack of interest in learning An efficient facility

of accommodation is particularly important because a greater period in school involves changing

focus between the chalkboard to near task such as writing at a desk Low values of

accommodative facility have been associated with symptoms related to near point asthenopia134

The result of the present study corresponds with findings by Evans et al 100 who reported that

dyslexics appeared to be slower at a test of accommodative facility Similar results were reported

in other studies 128898 but contrasts with results reported by Buzzelli 6 Buzzelli 6 assessed

accommodation facility using a Bernell Vectogram that utilized a polarised target to control for

suppression Buzzelli6 assessed accommodation facility as follows patients were asked to clear

the number 9 on the Bernell Vectogram target at 40cm They alternately viewed the target

through a 2D lens The test was continued for 20 cycles (a cycle being clearing the plus and

minus lens each presented once) The total time in seconds was recorded for the right eye left

eye and both eyes The polarized target controlled for suppression The use of the vectogram

to assess accommodation facility by Buzzelli6 differs from the flipper technique that was used in

the present study Secondly there was no control for suppression in the present study The

difference in results found between the two studies may be due to the lack of control for

suppression in the pre sent study Siderov and Johnston174 stated that ldquoclinical measurements of

binocular accommodation facility could vary depending on whether or not suppression has been

monitoredrdquo

A total of 317 of the poor readers in the study by Grisham et al 86 had inadequate (9cm or less)

accommodative facility

Allison173 suggested that there are different norms submitted by different authors for

accommodative facility measurements and that apart from adhering strictly to recommended

norms for accommodative facility testing other factors to be considered when evaluating

accommodative facility include the difficulty and speed of responses to the plus and minus lenses

122

and whether the subjects fatigue easily 173 Kedzia et al (cited in Zadnik)175 documented that

the factors that affect the testing of accommodative facility include the size and position of the

text being read the complexity of the target reaction time of the child to call out the symbol and

the magnification or minification factors introduced by the lenses themselves The above

variables are factors to be considered in assessing the variations in the findings for

accommodative facility reported in this study as compared to other studies

b Amplitude of Accommodation Accommodation is the ability to adjust the focus of the

eye by changing the shape of the crystalline lens to attain maximum sharpness of the retinal

image of an object of regard The absolute magnitude of the accommodative response is

termed the accommodative amplitude 120

Statistically the dyslexic group had a slightly reduced monocular amplitude of

accommodation (mean 12D) compared to the control group (1287D) but there was no

statistically significant difference between the two groups (p=070) The slight difference

in the mean amplitude of accommodation between the two groups may have been due to the

two subjects from the dyslexic group who had latent hyperopia

Similar to the present study lvarez and Puell88 reported that monocular accommodative

amplitude was significantly lower in the group of poor readers Evans et a l91100 in two

seperate studies 91100 reported that amplitude of accommodation was reduced in the

populations studied Ygge et al 102 and Goulandris et al 90 found no statistically significant

difference between the dyslexic and control groups Grisham et al 86 found that 247 of

the children had amplitude of accommodation which the authors classified as ldquoweakrdquo or

ldquovery weakrdquo while 115 had accommodative amplitudes classified as ldquoborderlinerdquo In the

present study the mean amplitude of accommodation for each group was within normal age

norms according to the amplitude norms and Hofsteterrsquos formula 106161

In the study by Metsing and Ferreira 103 a higher percentage of low amplitude of

accommodation for the right and left eyes respectively (516 and 533) was found in the

mainstream group compared to 291 and 288 in the learning disabled group The

123

relationships between the mainstream group and reduced amplitudes of accommodation of

the right (p=004) and left eyes (p = 0001) were found to be statistically significant

(plt005) Furthermore the relationship between the mainstream group and reduced ampli-

tude of accommodation of the left eyes (Cramerrsquos V = 0316) was found to be moderate and

that between the mainstream group and the right eyes (Cramerrsquos = 0286) to be low

When accommodation amplitude is assessed monocularly it measures the response for each

eye individually and the limiting factor is the magnitude of blur-driven accommodation

Monocular amplitude measures are particularly important to determine whether a patient

has accommodative insufficiency 107 The amplitude measured binocularly is usually

greater than the monocularly measured amplitude due to the influence of both the blur-

driven accommodative response and accommodative and vergence responses 120 175

Clinically the amplitude of accommodation is considered important mainly when it falls

below the expected age norm in which case the child may experience blur vision at near

Secondly a difference of up to 2D between the two eyes may also be considered clinically

significant 120

An inefficient accommodation function may lead to difficulties in learning as the focusing

system of the eyes play a major role in the learning process Children who suffer some

anomalies of accommodation are more prone to fatigue quickly and become inattentive than

those who have norrmal accommodation function 87 Symptoms of accommodation

insufficiency are specifically related to near vision work 176

Factors affecting the measurement of amplitude include differences when measurements

are taken monocularly as compared to when assessed binocularly the angle of gaze target

size age refractive error race and climate 175

c Accommodation Posture During near vision the eyes are not usually precisely focussed

on the object of regard but the accommodation lags a small amount behind the target If

the accommodation lag is small then the blur it causes is insignificant if high then it can

result in blurred print during reading 142

124

About 3928 (11 subjects) of dyslexic subjects had lag of accommodation compared to

4193 (13 subjects) of the control group The mean accommodation lag was similar in

both groups 092 for control and 088 for the dyslexic group There were no statistically

significant differences between the two groups right eye (p = 083) and left eye (p = 060)

Similarly Evans at al 100 reported that the mean accommodation lag did not show any

statistically significant difference between the dyslexic and control groups On the

contrary Metsing and Ferreira 103 found a high prevalence of lead of accommodation for

the mainstream group but a high prevalence of lag of accommodation in the learning

disabled group The relationships were found to be statistically significant (plt005)

between the mainstream and the learning disabled group and the lead and lag of

accommodation for the respective groups of the right and left eyes (p =000) In another

study Evans et al 91 reported a mean accommodation lag of + 112DS

An individual with a lag of accommodation habitually under accommodates and may lead

to difficulty with reading An accommodative response that manifests as an excessive lag

of accommodation may indicate latent hyperopia esophoria or may be associated with

accommodative insufficiency or accommodative spasm poor negative vergences or when

patient is overminused 177 The prevalence of high lag may be related to the prevalence of

latent hyperopia and esophoria in the present study

C Relative Accommodation (PRA NRA) The relative accommodation tests assess the

patientsrsquo ability to increase and decrease accommodation under binocular conditions when

the total convergence demand is constant67 It is also an indirect assessment of the

vergence system since the vergence demand remains constant while accommodative

demand varies 121 The results for the relative accommodation for all subjects from both

dyslexic and control groups were unexpectedly high The norm for the mean NRA and PRA

were given as approximately +2 or - 2 diopters from two reports consulted 119 121 while the

relative accommodation results in this study ranged between 3 and - 6 diopters Similarly a

higher relative accommodative value was reported by Chen et aland Abidin139 in a study of

vergence problems in Malay school children The only available study accessed that

125

reported on relative accommodation was conducted by lvarezand Puell 88 who reported

that the negative and positive relative accommodation values were similar in both groups of

children Statistically the results were (NRA Control group 19 06 poor readers 19

06 PRA Control group 23 09 Poor readers 20 13) compared to the findings from

the present study (NRA Control group was 311 047 NRA dyslexic group 322 079)

(PRA Control ndash 606 063 PRA dyslexic was -623 117) The p values were not

indicated

The higher values found in the present study may be due to the process of addition of trial

lenses specifically The phoropter has been reported to be a better technique to assess

relative accommodation 119

Although the relative accommodation was not assessed by many authors different opinions

were expressed regarding the assessment of relative accommodation Latvala et al 3 noted

that positive and negative relative accommodation ranges are ldquodifficult to take and

unreliablerdquo According to Morgan 178 the PRA and NRA results are often unusually high

but recommended that such tests results can be used as a sort of indicative way and can only

be interpreted loosely and suggested that the use should be restricted to special

circumstances Garcia and Fransisco 122 reported that high NRA can be associated with

disorders such as accommodative insufficiency and convergence excess while high values

of PRA are related to anomalies in which accommodative excess appears but concluded that

PRA and NRA findings were used mainly as complementary diagnostics tests of some

disorders in literatures consulted

A high value of the negative relative accommodation could indicate that the children were

exerting maximum accommodation effort Because 250D of accommodation is exerted at

the 40cm working distance the maximum amount of accommodation that would be

expected to relax accommodation at 40cm would be 250D A value greater than 250D

would mean that accommodation may not have been fully relaxed during the subjective

refraction 114 Generally a high value of the NRA may also mean that the refraction may

126

have been under corrected for hyperopia or over corrected for myopia 122 Garciaand

Francisco122 reported the relationship between high NRA values and under corrected

hyperopia to be due to the etiology of hyperopia

According to Grosvenor 114 the amplitude of accommodation is a limiting factor for the

expected value of the PRA For example ldquoIf a patient has an amplitude of accommodation

of only 150D we would expect the minus-lens-to-blur test to be no greater than -150Dhelliprdquo

A high value of the positive relative accommodation is related to convergence insufficiency 122 Illumination and the range of fusional vergence also affect the relative accommodation

An excessive illumination will give an erroneously high finding due to the increased depth

of focus 100

Dysfunctions of accommodation can significantly interfere with the comfort clarity speed

and accuracy of reading as the child develops reading skills 51 Wick and Hall 120 suggested

that the results for the tests of accommodation are more meaningful when analyzed

together as the results of individual accommodation function may not give a true reflection

of the childrsquos accommodation dysfunction

526 Fusional Reserves

Fusional vergence amplitudes is a clinical measurement of a person`s fusional vergence ability It

reflects the ability of the oculomotor system to maintain sensory fusion in spite of varying

vergence requirements 179 The horizontal vergence reserves describe the amount by which the

eyes can be converged or diverged whilst the subject maintains fusion 100161 It is a measure of

how much fusional vergence is available in reserve that can be used to overcome a phoria The

amount of base out prism required to produce diplopia is called positive fusional reserves

(measures convergence) 100 The measurements of convergence fusion are also referred to as the

lsquoamplitude of fusional reserves 111 Clinically the fusional amplitude provides information about

a patientrsquos ability to maintain comfortable binocular vision 100112

The participants from both the dyslexic and control groups either could not report or understand

127

blur so the result for break and recovery was used in all analysis of vergence function Evans et

al 100 reported similar observations For the fusional reserves only the findings for the near

positive fusional vergence (convergence) are emphasized The positive fusional

reservesvergence (measures convergence ability) may be more important in assessing reading

dysfunction More so it has been suggested by other authors 112121180 that near measurements

are more relevant in assessing vision functions in dyslexic children

Based on Bishops 111 recommended norm of between 30 and 40 prism diopters for positive

fusional amplitude at near the prevalence of abnormal positive fusional vergence at near was

83 for the control and 74 for the dyslexics This appears unusually high Walters 168

explained that a possible reason for an unexpected high base out finding is that exophoric

conditions makes compensating for base out more difficult than base in testing It is unclear if

this is the case in the present study although the prevalence of exophoria at near in the present

study was 95 The compensation for a phoria is maintained by fusion reflex so knowing the

magnitude of the vergence amplitude that is needed to compensate a given phoria is very useful

clinically181

For the present study at near the negative fusional vergence (base in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base out

vergences) was similar for both groups Overall there was no statistically significant difference

between the groups (p = 021 for BI break at near) and (p = 084 for BO break at near) Evans et

al 100 reported that the dyslexics have reduced fusional reserves compared to the control group

Bucci et al 89 reported reduced divergence capabilities in dyslexics at near

When assessing fusional reserves the blur point is a function of the flexibility between vergence

and accommodation When little flexibility exists the addition of even low prism causes a

simultaneous accommodation shift with resultant blur Lack of low blur findings may indicate an

inadequate flexibility bond between accommodation and convergence The break point is an

indication of the quality of binocular function Break values will be adequate when fusion ability

is good while the recovery measure is a more subtle indicator of the quality of binocular function

Consequently reduced blur break or recovery findings indicate the presence of near point stress

128

182 Some subjects from both groups had break values higher than 40 prism diopters for base in

and base out reserves Wesson et al 125 in their study reported a similar observation on objective

testing of vergence ranges Such high break findings may be because suppression was not

controlled while assessing the fusional reserves in the present study When suppression is

controlled the average vergence values will be lower because the test is stopped when the

suppression is detected If suppression is not monitored the break is not detected until the

stimulus is outside the suppression zone and a higher vergence value is obtained 125 However it

is important to note that the significance of such suppression in binocular individuals is unknown 126 Again in special cases such as suppression large abnormal findings (compared to

normative ranges) may permit better functions in reading than a lower abnormal finding 183 For

example an individual who totally suppresses the vision of one eye is less apt to have difficulty

at reading than the individual who only partially suppresses the vision of one eye 183

Ygge et al102 assessed vergence fusion using the prism bar They reported that the fusion

convergence and divergence capacities at distance and near were similar in the two groups The

mean fusional convergence capacities at distance were 1680 prism dopters for both the control

and the dyslexic group At near the corresponding figures were 2640 prism diopters and 2670

prism diopters respectively The mean fusional divergence capacity at distance was 650 prism

diopeters and 620 prism diopters in the dyslexic and control groups respectively whereas at near

the fusion divergence capacity was 1050 and 1020 prism diopters respectively There was no

statistically significant difference between the groups

Latvala et al 3 reported a prevalence of fusional amplitude greater than or equal to 32 prism

diopters to be 61 for the control group and 75 for the dyslexic group using referral criteria

of 32 prism diopters at a distance of 33cm At near a testing distance of 40cm was used in this

study The difference in prevalence between the two studies might be from the different test

distances used Furthermore as detailed by Rouse et al 158 large intra subject intra-examiner

and inter-examiner variations in the assessment of fusional reserves have been reported

Vergence amplitudes have been reported to vary with alertness that is whether the subject is

tired or rested or under the influence of a toxic agent 184

129

Bedwell et al 171 reported that neither convergence nor divergence were significantly related to

reading difficultyrdquo Similarly Metsing and Ferreira103 found no statistically significant

difference between fusional vergences amplitude (Base-out base-in) at near No details were

provided

There are several limitations with the assessment of fusional reserves Sheedy (cited by Rouse et

al) 158 remarked that ldquo the general opinion is that when fusional vergence tests are repeated on

the same patient the second value found may be quite different from the first a difference of 10

PD from one fusional vergence amplitude measurement to another is not unusual unless rigorous

controls ar a typical difference of 3- 4 prism diopters but can measure differences as large as 12

prism diopter on follow up visit and that inter-examiner variation can be as large as 10 to 16

prism diopters The authors concluded that the positive fusional vergences difference could be

due to children having difficulty understanding the instructions or expected endpoints children

being slower responders or may be poorer observers The blur readings on the vergence reserves

for both distance and near were excluded in the analysis as it was difficult to get the children to

elicit a proper response It is possible that the children did not understand the instruction or the

concept of blur despite repeated explanations and trial readings This variable is often difficult

for young subjects to understand Similar problems were encountered by Evans et al 100 who

commented that several subjects were unable to appreciate a blurrdquo Scheiman et al 126 stated

that ldquowe were unable to consistently obtain a blur finding from our samplerdquo and that rdquo less

than13 of the subjects were able to report a blur we therefore only recorded the break and

recovery findingsrdquo

Wesson et al125 emphasized that the ldquomajor limitations of any vergence measurement

isrdquohellipnecessity to obtain a subjective response to blur break and recovery and that working with

children whose responses are not reliable is difficultrdquo It was suggested that a break and recovery

point could be determined by observing the subjects eye movements as prism power is increased

This technique was applied in this study However most children have difficulty maintaining

fixation long enough to measure vergence ranges

Vergence is influenced by several factors including awareness of the distance of the object

130

(proximal vergence)cross-linking with the accommodative system (acommodative vergence) and

the fine tuning of ocular alignment during the fusion of each monocular image into a single

percept (fusional vergence) 161 and other factors such as test target and lighting conditions 125

Generally in the assessment of a patientrsquos binocular status the fusional reserves may be more

useful when analyzed with the phoria measurements 115

The vergence system is closely related to the accommodative system and symptoms may at times

appear similar The symptoms associated with deficiencies with the vergence system include

letters or words appear to float or move around postural changes noted when working at a desk

difficulty aligning columns of numbers intermittent diplopia at either distance or near 185

Furthermore ldquodysfunctional vergence could cause an excess of eye movements especially of

small saccades The ability to continue the rapid decoding of the visual characters decreases as

more stress is placed on the vergence mechanism of the dyslexic during reading which makes

them tire more quickly than normal readersrdquo 6

A possible explanation for the relationship between vergence control and reading proposed by

Riddel et al (cited by Morad et al) 186 is that children with poor vergence control had impaired

accuracy of spatial localization that may impend their ability to accurately determine the position

of letters within words

527 Ocular Pathology

Two participants from the dyslexic group had cataracts Only the study by Hoffamn 83 reported

that one child had conjunctivitis

54 Summary

The results of this study do not vary significantly from the international studies in spite of the

differences in study designs tools and sample sizes Although studies have indicated differences

131

in the prevalance of open angle glaucoma and myopia between Caucasian and African

populations this study does not indicate a specific difference in the prevalance of vision variables

between Caucasian and African dyslexic school children However the lack of studies on

African children makes it difficult to reach any conclusions regarding structural differences that

may affect the presence of dyslexia Comparing the findings of this study with those done

internationally highlights the complexity of investigating the visual conditions that may be

associated with dyslexia and the need for rigorous and consistent testing methods in order to be

able to compare results

Having completed a discussion of the study findings in Chapter Five Chapter Six will present the

study conclusion and indicate its limitation as well as recommendations for future studies

132

CHAPTER SIX CONCLUSION

61 Introduction

Being able to read and write is an essential part of being able to interact with the world As

vision plays a major role in reading and the learning process a normally working visual system is

essential for efficient reading

With as many as 20 of Caucasian children being affected to a greater or lesser degree by

dyslexia 1631 there is a need to identify its cause and develop remedial actions to reduce its

effects As studies 4 59 have shown that intellectual ability is not the cause of dyslexia research

to enable people to reach their full potential in spite of their impaired reading ability requires a

multidisciplinary approach of which optometrists are part

This study aimed to determine the prevalence of vision conditions in a South African population

of African dyslexic children and to study the relationship between vision and dyslexia in an

African setting This was done by investigating visual acuity refraction and binocular functions

between two groups of 31 African school children one group attending a school for children with

learning difficulties and the other a mainstream school This enabled a study of possible vision

defects in African dyslexic children as well as a comparison between the two groups

The study targeted African school children its objectives being to

1 determine the distribution of visual acuity disorders among dyslexic children

2 determine the distribution of refractive errors among dyslexic children

3 determine the distribution of heterophorias among dyslexic children

4 determine the distribution of strabismus among dyslexic children

5 determine the distribution of accommodation disorders among dyslexic children

6 determine the distribution of vergence disorders among dyslexic children

7 determine the distribution of ocular pathology among dyslexic population

133

8 compare these findings to a similar group of non-dyslexic children

A review of the literature showed that most studies that investigated visual functions in dyslexic

school children were conducted on Caucasian population and that the findings of these studies

were inconclusive The broad areas of vision investigated in the previous studies 2-6 87-89 were

visual acuity and refraction binocular vision and ocular pathology

As studies conducted on the African population was lacking this study examined visual acuity

(using LogMar charts) refraction (static retinoscopy without cycloplegia) near point of

convergence (RAF rule) [accommodation functions 2 flipper lenses to assess accommodation

facility MEM retinoscopy for accommodation posture and pushndashup method to evaluate

amplitude of accommodation] ocular alignment (cover test and Maddox Wing) and strabismus

(Hirschberg test) and fusional reserves (using prism bars)

The study was conducted on a African population of 31 dyslexic school children selected from a

school for children with learning difficulties and 31 control participants from a mainstream

school in Durban Their ages ranged between 10-15 years The participants were selected using

the convenient sampling method as there were only a few participants classified as being

dyslexics All data collection procedures were conducted at the respective schools

The prevalence of vision conditions were presented in percentages () Data was analyzed using

the Statistical Package for Social Sciences (SPSS) and was expressed as mean (M) and standard

deviation (SD) and calculated for descriptive purposes at 95 confidence interval (CI) The level

of significance considered to support a hypothesis was taken as P lt 005 For comparison all

data from both groups was subjected to a two sample t test (2-tailed)

134

62 Summary of findingsIf an association between visual function and dyslexia exists a higher prevalence of vision

defects in dyslexic children than in the non-dyslexic participants (control group) would be

expected

621 Visual Acuity

The prevalence of visual acuity worse than 69 (23 dyslexic and 225 control) is the same in

both the dyslexic and the control group and there was no statistically significant difference (p =029 023 right and left eye respectively This is similar to the findings by Buzzelli 6 and

Goulandris et al 90 which found no statistically significant difference between the two groups

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexia and the control group (p=029 for the right eye and 023 for the left eye)

622 Refractive Errors

The prevalence of refractive errors was similar in both groups There was no statistically

significant difference between the two groups (p=066 for the right eye and 092 for the left eye)

This is similar to the findings by Evans et al 2 which found no statistically significant difference

between the two groups (p=058)

The null hypothesis was therefore accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 066 for the right eye and 092 for the left eye)

a Hyperopia Hyperopia was more prevalent in the dyslexic group than in the control group

This is consistent with findings in other studies 88 93 95

b Myopia The prevalence of myopia was the same in both groups This is similar to the

findings by Eames 95 which found no difference in the prevalence of myopia between the

two groups

135

c Astigmatism Astigmatism was more prevalent in the control group than in the dyslexic

group This is similar to the findings by Alvarez et al 88 which found a higher prevalence of

astigmatism in the control group than in the dyslexic group

d Anisometropia The prevalence of anisometropia was the same in both groups (2 subjects

each) This is comparable to results reported by Latvala et al 3 where the prevalence of

anisometropia was 36 (2 subjects) for the dyslexic group and 6 (3 subjects) for the

control group

e Amblyopia Amblyopia was more prevalent in the dyslexic group compared to the control

group This is similar to the findings by Latvala et al 3 and Rosner and Rosner 93 which

found a higher prevalence of astigmatism in the dyslexic than in the control group

623 Near Point of Convergence

The prevalence of remote NPC (ge10cm) was higher in the control group than in the dyslexic

group and there was a statistically significant difference between the two groups (p= 0049) This

is similar to the findings by Kapoula et al 87 Latvala et al 3 and Evans et al 91 who found a

statistically significant difference between the dyslexic and the control groups on the NPC

functions

The null hypothesis was rejected as there was a statistically significant difference between the

dyslexic and control groups (p = 0049)

624 Heterophoria

The prevalence of exophoria at near was higher in the dyslexic group (95) than in the control

group with no exophoria There was no statistically significant difference between the two

groups (p= 059) The prevalence of esophoria in the dyslexic group was higher than that found

136

in the control group and there was no statistically significant difference (p =046) This is similar

to findings in other studies 3 12101163 which found no statistically significant difference between

the two groups

The null hypothesis was accepted as there was no statistically significant difference between the

dyslexic and control groups (p=059 for exophoria) and (p=046 for esophoria)

625 Accommodation Functions

a Amplitude of Accommodation The dyslexic group had a reduced monocular amplitude

of accommodation compared to the control group but there was no statistically significant

difference between the two groups (right eye p = 007 left eye p=022) This is similar

to the findings reported in other studies 3 87 91102 which found no statistically significant

difference between the two groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation amplitude (right eye

p=007 left eye p=022)

b Accommodation Facility The participants from the dyslexic group performed

significantly worse than the control group in accommodative facility function and there

was a significant difference between the two groups (p=0027) This result corroborates

findings reported in other studies 12 88 98100 which found a statistically significant

difference between the two groups

The null hypothesis was rejected as there was a statistically significant difference between

the dyslexic and control group in accommodation facility (p=0027)

c Accommodation Lag The prevalence of lag of accommodation was higher in the control

group compared to the dyslexic group and there was no statistically significant difference

There were no statistically significant difference between the two groups (right eye p=083

137

and left eye p= 060) This is similar to the findings by Evans et al 100 which found no

statistically significant difference between the two groups The mean accommodation lag in

the present study were 092D for control and 088D for the dyslexic which is comparable to

reports by Evans et al 91 who reported a mean accommodation lag of + 112D

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and control groups in accommodation posture

d Relative Accommodation (PRA NRA) The mean values for the relative

accommodation are similar in both groups and there was no statistically significant

difference between the groups (p=068 for NRA and p =051 for PRA) This is similar

to the findings reported Alvarez and Puell 88 which found no statistically significant

difference between the two groups in relative accommodation

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups in relative accommodation

626 Fusional Reserves

For the present study at near the negative fusional vergence (base-in vergences) was reduced in

the dyslexics compared to the control group while the positive fusional vergence (base-out

vergences) was similar for both groups There was no statistically significant difference between

the groups (p= 021 for BI break at near) and (p = 084 for BO break at near) This is similar to

the findings by Evans et al100 which found no statistically significant difference between the two

groups

The null hypothesis was accepted as there was no statistically significant difference

between the dyslexic and the control groups (p = 021 for BI break at near) and (p = 084 for BO break at near)

627 Ocular Pathology

138

Two participants from the dyslexic group had cataracts

63 Implications of the Study Findings

a Causal Relationship This study presented the prevalence of some vision defects in a

South African African population of dyslexic school children as compared to a population

of non-dyslexic children and could not establish any causal relationship between vision and

dyslexia The vision parameters that showed that statistical significance may only be

reflective of factors associated with dyslexia but in a non-causal way This may mean that

the prevalence of the particular visual variable is much higher in the dyslexic population

than in the control group Vision is essential for reading but the vision anomalies that are

found to be associated with dyslexia also occur in children who do not have reading

difficulties It is therefore possible that other complex vision functions may be more related

to the etiology of dyslexia than peripheral visual factors such as NPC and accommodative

facility which showed statistically significant differences in the present study

b Risk Identification The findings of this study also suggest that dyslexic children are not

at more risk for the identified vision condition compared to non-dyslexic children

However even when vision defects may not be the cause of dyslexia the presence of

uncompensated vision defect in a dyslexic child may constitute difficulty while reading

and should be compensated for Improvement in vision will make reading easier for the

dyslexic child

64 The Study Strengths

The study methodology was based on reports found in other studies and consisted of the

following

a Classification The subjects selected for the dyslexic group represent a dyslexic population

that was classified according to the criteria used in the study by Latvala et al 3 and Evans et

139

al 100 The study was undertaken at a school for learning disabled learners and all students

were assessed and categorized as being dyslexic by the psychologists

b Study Examiner Examination of the subjects was conducted by the same optometrist who

had some experience in paediatric optometry This minimised the possibility of bias and

inter-examiner variability

c Examination Techniques The examination techniques used in this study were the same

as the techniques used in major referenced studies which were published in peer-reviewed

journals

d International Relevance It is the first study that has assessed broad vision functions in

a dyslexic population in an African setting

6 5 Limitations of the Study

The factors which may limit the generalization of findings of the study are as follows

a Inattentiveness This occurred in some children after numerous tests and the possibilities

of their lack of understanding of certain instructions However this problem was

minimized as trial readings were taken for some procedures and the participants were

instructed accordingly As stated in Section 34 testing was discontinued when the child

was tired This approach helped to minimize fatigue which could have negatively

affected the results In addition objective techniques that required minimal responses

from the participants were utilized in several procedures such as the cover test

retinoscopy and the monocular estimation method for evaluating accommodation posture

b Convenience Sampling This method of sampling was used given the limited number of

available subjects from the target population The available studies 235687-91102103 on

the subject of dyslexia learning disabilities and vision also used the convenience

sampling method

140

c Generalised Findings The generalization of the findings of this study is limited by the

fact that the refraction data was collected without the use of cycloplegia At the inception

of the data collection the diagnostic drug usage by optometrists was not approved in the

scope of practice in South Africa

d Sample Size The sample size was small as there was only one school for dyslexic

African children in Durban at the time of this study However review of the literature

also reveals that the sample sizes on studies conducted on dyslexia learning disabilities

and vision were typically small The targeted sample size of one hundred for this study

could not be met due to the limited number of subjects available The average sample size

of eight published studies 356 8790100 102 conducted on dyslexic children was 41

e Suppression Control The assessment of binocular accommodation facility and the

vergence reserves were performed without controlling for suppression

f Assessment of Relative Accommodation the use of a Phoropter would have yielded a

more conclusive result

g Eye Movements these were not assessed

Despite the acknowledged limitations inherent in this study the study provides useful

information and a research perspective on the prevalence of vision defects in a South African

population of dyslexic children which has not been conducted before

66 Recommendations

The following recommendations are made based on the findings of this study

661 Future Research

141

1 The same study with a similar protocol larger sample size with randomized sampling

2 Investigate the relative accommodation in in both groups using the phoropter

3 Assess the binocular functions with suppression control

4 Assess refractive error under cycloplgia

662 General Recommendations

1 All dyslexic children should have their eyes examined routinely to rule out

possibility of vision defects impacting on their reading performance

2 The Department of Education be reminded that visual defects can impact negatively on

the reading ability of learners and that teachers need to be more aware of pupils who may

present with signs of visual problems such as holding book too close or battling to see

the chalk board

3 The principals of the mainstream school should also be informed that vision defects were

detected in the non-dyslexic children

67 Conclusion

As a neurological condition which manifests primarily as a language-based disorder with

difficulty with words dyslexia requires a multi-disciplinary approach to its management with

an eye examination being needed to eliminate any possible effects that ocular conditions may

contribute to reading problems

The study provided the prevalence of disorders of visual acuity and refractive errors near point

of convergence accommodation dysfunction heterophoria strabismus and fusional reserves

among African dyslexic children of age range 10-15 years The only vision variable that was

more prevalent and that is significantly associated with dyslexia was the binocular

accommodation facility while only the near point of convergence (break and recovery) was

significantly more prevalent in the control than the dyslexic group The existence of these

statistically significant differences between the two groups may not imply clinical relevance

due to the small sample size The comparison of vision characteristics between the dyslexic

142

and control group indicates that the dyslexic children are not more at risk of having these

vision condition than the non-dyslexic children although sample size was small

Further research on African children will add to the body of knowledge about this group with

respect to the relationship between vision conditions and dyslexia about which very little is

known Only a few studies have provided evidence of the extent of dyslexia among African

children yet even a small percentage could result in many millions of children being affected

given the population of Africa While the problems that result from dyslexia are not life

threatening they do affect peoplersquos opportunities and quality of life

Unfortunately many African countries do not have the resources to provide specialized

schools for children with learning disabilities and every effort therefore needs to be made to

correctly identify the problems and possible causes of dyslexia In order for African countries

to implement the strategies to meet their millennium development education goals of ensuring

that by 2015 all children will be able to complete a full course of primary schooling they

need to provide not only for children in mainstream schools They need also to plan for

children whose academic ability would be enhanced by the appropriate diagnosis of learning

difficulties the provision of a simple eye test and spectacles or corrective apparatus where

appropriate

Dyslexia is not a disease for which a cure can be found rather long-term sustainable

interventions need to be put in place to ensure that the problems are identified and appropriate

remedial action is taken to minimize the impact it has on their life While many African

countries may not be able to afford schools for children with learning disabilities greater

awareness of the manifestation of the condition needs to be made to all teachers in an effort to

provide these children with the possibility of staying in main stream schools for as long as

possible where there are no alternatives With this in mind it is anticipated that this study will

contribute to the body of knowledge on vision conditions of African dyslexic school children

143

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